I have seen these ones with a half moon shaped pad that fits your leg nicely - they look pretty good, anyone use one?
When I - infrequently! - fly with a kneeboard, I use the type marketed as the
"Pilot's Pal". It is sold by Sporty's, but I purchased mine from a local pilot supply shop. The bottom is concave and it does a pretty good job of stating put, except for aerobatics.
It's simple, sturdy, and compact, and not a bad choice if you really need a kneeboard. My only complaint is that the pen/pencil holder should be in a horizontal position at the top, rather than a diagonal position on the top right corner: the latter causes both ends of your pen to sit out in the breeze, so it is vulnerable to being knocked off if you are in a moderately cramped cockpit. However, this can easily be fixed with a pair of pliers and a drop or two of glue.