This is the one I was going to post (also my favorite!) - except when I do it, the leg that you say you put on the crotch - I keep that as a butterfly hook and then wrap the my other leg around the opponent's extended leg. Also one of the upa variations I do is I try and bridge hard to get the opponent to buck forward so that they have to post with both hands, then I reach up through and overhook around one arm and his body. The problem with this is if you don't do it fast enough and if your opponent regains balance quickly you're looking at an armbar - either from high mount or the standard arm bar setup. Or if your opponent is an Eddie Bravo fan, you're looking at Monkey Mount. Simplest one that I use sometimes in conjuction with the other ones is to just stick one elbow against same side knee (ie your left to opponent's right knee) and then stick the opposite side knee (ie your right) up against his butt. Hip bump and slide the other leg under for half guard (this would be your left). Someone posted something like this before, except using one (in this case right leg) leg to hook the opponent's ankle.