TOAD
The ones I have came from 3 Rivers.
They take a 21/64 bit. There is a lip on them that stays above the surface of the wood, but you could counter sink them to make them flush.
Once the holes are drilled you screw them in all the way with a flat bladed screwdriver. Once all the way down, you screw in the bolt all the way. The bolt goes in easy until you hit bottom. Once it bottems out, screw in hard. This spreads the bottom of the insert and it bites into the wood. It is a very easy installation, and looks like a factory insert when it is done.
Lots more attractive than just a hole drilled.
Two words of caution. I put some on a Diamond Annivershery Black Widow. I was not paying attention and drilled one hole all the way through the riser. Another thing to watch is when drilling the holes make sure they are absolutely straight. If not, the insert ends up a bit crooked.