Thinner than you expect but works well - VISM AK Featureless Grip

First thing's first. The shelf is perfect. Just the right size, shape, and location.
The rest of the grip is surprisingly thin and hard even at the texture sections.
When you have your hand on it with the thumb in place, it keeps your hand shaped exactly like you're about to start a thumb war game.
Your thumb is up high, pressing down and wedging the shelf into the curve, which leaves your 4 other fingers perfectly pressing up against the left side of the grip textures as counter pressure to lock it all in.

It's a little uncomfortable at first for your thumb but ultimately feel like a much better hold of the grip than having no shelf at all.