Technology is here and now, I've flown couple of SEP aircraft that have round transponders that would easily fit in ANY glider aircraft. What I think is the problem with gliding, the people rather spend thousands of euros on brand new GPS than buying the most simple C-mode transponder
1. It won't just fit in many, probably most gliders. Look inside one - space is extremely limited, especially in the panel. Which instrument must I throw out to make space?
2. Weight and balance is also critical, and most units are too heavy.
3. Where are the extra batteries for the power supply (no generator remember) to go?
4. In the UK, we are only allowed to fit a Mode S transponder, and there is currently only one model which is remotely possible in terms of weight and power consumption.
5. Under EASA rules we can only fit a transponder in accordance with the manufacturer's approved scheme. There is no such scheme for most gliders.
In my 1968 glider I happen to have space to fit 7ah worth of 12v DC power, so for me it's only the other 4 problems that make it impossible. Plus the cost, which I reckon to be around 25% of hull value.