The minimum entry qualification for a diploma is a pass in class ten and later one has to appear in the competitive tests conducted by the state board of technical education. The total duration of the diploma course, irrespective of the streams is three years.
Hence. if you think of the other option by considering your present qualification being class twelve, you can continue the degree course in Engineering taking one additional year only in the completion of the degree course if you calculate the duration of the diploma course. So taking admission in engineering college would be much better.
If at all, you are interested for the diploma course in any stream, you should have to clear JEXPO and on the basis of your scoring, you will be eligible for admission in the Government Polytechnic including the private one.
The other alternative is to seek admission in a private polytechnic run by different state governments such as Orissa, Tamilnadu, Karnataka etc., with the management fee and even if you pay the huge fees for a private institution, you may not get a suitable placement.