Man I missed part of the questions..sorry
To build them up I use cork gasket material. Comes in rolls about 1/8" thick maybe? Get it at auto supply stores. Cut it with a razor or scissors. Glue on with contact cement where you need it (mulitlayer where you want like at the back) and it can be shaped with a file, then covered with leather. Another advantage is the cork absorbs some of the shock/vibration felt thru the handle. Kinda smells funny for awhile though, but it goes away.