This worked pretty slick for me. On one of my Dwyer longbows, I felt the grip was a little small.
So I purchased some adhesive backed cork shelf liner. Cut the pieces in modified triangular shapes wrapping from back to front to build up the heel. Then built up a small thumb rest. Then I took a rasp to the cork and finally a little sanding.
Then I completed by putting a sure-grip over the whole thing. Not only did it work great, it provided a nice warm cusion without being "mushy"