I've got an L-shaped desk with a rounded inside corner, which is where my keyboard sits, and the wrist rest then hangs off the desk partly.
Given that for 3/4 of the keyboard's width, the wrist rest is off the desk, all the rubber-backed rests are no good because they sag under pressure.
So, I'm looking for
this, basically. However, I'm not big on leather because it tends to get hotter than fabric.
So: Reinforced, preferably with a metal plate, fabric-covered, preferably memory foam over gel.
Right now I've got a hand towel folded over itself, sitting on cardboard, but it's less than ideal. Likewise, I think sitting a normal wrist rest on cardboard would make it too tall.