I went scrounging around at the hardware store today and came home with a fist full of goodies.

The neoprene washer works well if it is thin and the hole is small. Pull it on from the tip end of the butt blank and it will roll up from the blank to the grip very nicely. I still don't know how it will do with the Flex Coat, though.

I found some black beveled faucet washers that work out great! You do not have to be precise with the hole size as they stretch some. In fact, if the hole is a couple hundredths undersized, the washer winds up looking like it was made for the blank. 3/8 to 1/2" washers are good places to start size wise. The ones I found at Ace Hardware are a nice shiny black.

The last thing I tried was stainless steel washers. I put a couple in my doming block, beat the snot out of them and polished them. The center hole needs to be reamed to fit the blank after doming. These make nice checks. It's worth playing with some more to see what shapes I can get out of this. These first two are convex. I have an idea to get concave fillet looking checks, too.