I don't believe they make the proper chambering for a passable self-defense deringer. The Semmerling comes close, but it is expensive, if you can find one. There is also a single-shot (45ACP Downsizer) that is a BEAR to shoot, but I don't know if they are still available, either. A replica of the WWII "Liberator" might be handy.
If I were able to start from scratch, I would make a wildcat cartridge. I would go for the shortest case that could contain enough of a high energy-dense powder to deliver the largest diameter bullet I could hide on my person (the bullet diameter will determine the width of the gun).
The shortest case will allow the maximum barrel length for the longest possible acceleration time within a given overall firearm length.
So, take a 500 S&W, 45 ACP or 45 Colt cartridge case, cut the length to match the depth of Bullseye powder that delivers maximum pressure. Ream the inside of the case to proper diameter if necessary). That means a short casing.
The .45GAP comes to mind if you are not inclined to modify your brass, but don't EVER let an adequately loaded 45 GAP cartridge get near a stock Glock.
I would be inclined to take 45 ACP brass, cut to a dimension shorter than 45GAP, and have a 45 ACP deringer rebarrelled to fit that brass. Then load it with an energy-dense powder and watch for pressure signs as I worked up to maximum velocities or 100% density.
I would also tr to find a grip/handle that gave my hand enough to hold onto without being impossible to hide. A spring-powered folding handle is something I have never seen, but if my life depended on a deep-concealment two-shot hideout gun, I would not spare the expense.
Lost Sheep