See and avoid
I've got a Flarm in my glider. I can't run a transponder, all the battery space and power is claimed by the vario, radio, and Flarm. There isn't either room or electricity or indeed money for a transponder. I use my eyes, aided by the Flarm and a bubble canopy. Now, if you are worried about flying in class E instead of grabbing more airspace look at the possibility of getting Flarm certificated for your use, as what it will show you is gliders! We don't want a midair any more than you do. Or use the airports that are already in class D airspace. Not that that will help glider pilots beaten out of the sky by light aircraft, which is statistically far more likely.