I've also recently made a siamese cube, and honestly, you don't need to make custom corner connectors. The edge is what really connects the cubes, the corners are only supposed to provide stability. Originally, I had just regular corners there, nothing added whatsoever. A few days ago, I had the insight of glueing the two corner connectors to the edge connector, as said on Ton's guide, except I didn't really mod the corners. This seems to be working for me, at least over the last few days.
-Alexei