I use the same thing my wife uses in the kitchen for cutting up
veggies. It's a plastic (maybe PVC) board with a built in hole
for a handle. I think she got them at Wal-Mart for a couple
bucks. It doesn't fit in any rod holder but since it's flat it fits
just about anywhere. Thanks to the built-in handle, I can
dip it in the water and clean it off easily.
Oh, by the way, they come in two or three different sizes.