Looking for guidance wrt to diploma courses

You already had MTech in Structural Engineering. You have very good chances to get selected as a lecturer in Engineering colleges and Universities. You apply for the NET examination and clear that so that the chances of becoming a lecturer in government colleges will improve and you will get UGC scales also.
Alternatively, you can get a fellowship and go for a PhD in any good University under an eminent professor and you can work for PhD. These days the SRF amount is also very good and you can manage easily with that amount.
Instead of going for a certificate course you can try this way.
You can do CAD/CAM course that will add to your credentials and you will have more chances of getting a job. If you are planning to go to foreign countries, You can do a diploma in foreign languages which will add credit to you and your application will be given preference over others who don't have this certificate.

You are a civil engineering graduate with a post graduation in structural engineering and that itself is a good qualification. You can try for job in big infra structure projects and can apply in companies like L & T, ABB, Hindustan Engineering etc which are engaged in advanced infra structure projects.

Anyway if you are not seeing much scope or interest in this particular line then there are other avenues which you can consider by going for short certificate or diploma courses. Some of these are Environment Engineering, Geotechnical Engineering, Sewage and Effluent Treatment Technologies, CAD/ CAM, Consulting Civil Engineering, Water Engineering, Design Engineering etc. There are many institutes where the short courses or diploma for these specialized branches are carried and depending upon your interest I can suggest some of them as below -

I. Indian Institute of Water Management, Bhubaneswar.
2. International Institute of Waste Management, Bhopal.
3. Indian Institute of Environment Management, Mumbai.
4. National Institute of Design, Ahmedabad.

Structural engineers have huge demand even in the present market because of their distinguished jobs in the different domains such as Railways and Tunnel Engineering, Recycling of wastes, Resolving the environmental issues, Public Health Engineering etc. Being a M.Tech in structural - engineering can offer you a chance in a government engineering college by clearing the NET test conducted by CBSE. You have to ensure that your aggregate marks in the said examination is not below than 50 percent otherwise you will be disqualified for the test.
Recruitment would follow in order of merit in respect of your scorings in the test and hence you need to have a thorough preparation.
The other possibility to enhance your qualification is to take up the doctorate degree under the guidance of the eminent professor of the university and he will assign you you a thesis and you need to embark independent readers in the similar line. You need to publish your research - work in a reputed journal and the same has to be approved by the scholars including your guide. Adding a Ph.D degree in the structural - engineering will open up your avenues in both teaching line in an engineering college or your inclusion in the prestigious Research - work.
Some short courses diploma in the following lines would also be advantageous such as
1) A diploma in CAD/ CAM course.
2) To undertake a diploma in the Russian Language/ German language.
3) To prepare for GRE or TOFEL so as to widen your chance for jobs in the foreign countries.

After M Tech a diploma would add little unless it is in a niche area. The jobs for freshers after B.Tech or M.Tech are a little challenging because of the increasing competition. Companies look for people with 2-3 years experience, I would suggest focus on this and then you can decide on a diploma course.

What most would do is to work as a RCC structural Design consultant either on their own or in a reputed brand. You can explore the options of innovative building structures, green homes, eco-friendly industrial constructions, water management etc.

Please focus on firms like Shapoorji Pallonji, Aarvi Encon,Thronton Tomasetti, LERA, Besten,Skytreeconsulting etc. You can also focus on L&T, MECON, all state and central metro,roadways,elevated railway sectors.Even look at offshore & shipping industry related jobs initially for the career to start off.

Lot of people focus on CAD courses but having this alone does not make you a wanted strcutural engineer. What you need is some experience on the CV from the well known companies or individual consultants.
Some engineers focus on STAAD pro(structural analysis and design software courses).

Have you considered registration in the Indian Society of Structural Engineers or Indian Association of Structural Engineers. Once you interact with your peers and well known names, you would develop some contacts that could open doors for you. You can participate in the Innovative structural design and engineering competitions that would add value to your CV.

Lastly you can consider the options mentioned by above members about a career in teaching or a PhD. IGNOU has certificate courses in Energy Technology and management, Competency in Power Distribution management, see this interests you.

I am not able to understand your requirement for Diploma. Is it like you are not getting a good job? You are a postgraduate in a very good domain and if you try, you will be able to get a good job.
Apart from the Diploma, you can have below options to have an excellent career-
1. Appear or try for CSIR-UGC NET or Lecturership examinations. By this, you can get good teaching options in government colleges with excellent packages.
You can also go for a Ph.D. with a stipend. Both of these options are quite good in your field.
2. If you are not looking for a teaching or research-based career, you can try in related companies for a job. They may offer you a short term training as you will be fresher and then they will offer you a designation with a suitable pay scale.

3. I am not sure but is it like you have not scored good grades in M.Tech and that's the reason you want to pursue a Diploma? If it is the case, then you can consider the short term courses offered by some registered firms as they give job placement guarantee.

Still, if you want to pursue a Diploma, please note that no Diploma has a higher probability for jobs than Postgraduation degree. You may consider your current institute to pursue Diploma, as you will not need to put efforts for admissions formalities and all.