There’s the Mattias half-keyboard, which has a mirrored layout. Someone posted a mirrored layout in a thread, too. I’m borrowing a UHK right now, and I tried using a mirrored layout, but I don’t find mirroring intuitive. That may be because I’ve always typed with just one hand. As I type this message, I’m using a half-keyboard layout that translates the left half of the keyboard onto the right side without reversing the order of the keys. For me, this is a lot better than mirroring.
But, I’m going to go back to what’s worked best for me, which is a full QWERTY layout with a one-handed typing method.
I once tried a contraption called the MALTRON single handed keyboard, which is very different than all the layouts I just described.