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.