I hold the sandpaper on the flat on my palm. That gives a convex edge. If I want something more fancy, I'll either use two sided tape and stick the paper to a piece of plastic or whatever. I've used my machete as a sharpening support countless times too. Works well. Best thing for a clean convex edge is the machete plus a fwe square inches of camping matt closed cell foam. That's great.

Just find the trick that does it for you <img src="/images/graemlins/wink.gif" alt="" />

David