I made up this AMUB-2 with a walnut-faced pickguard. This was plain-back walnut veneer epoxied down onto 1/16" black Garolite sheet. The Garolite is really strong structurally, and is easily machined.
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If you want to make your walnut into a removable "pickguard", you need to reinforce the back somehow. Epoxy it down to a Garolite sheet, or an aluminum sheet. Or, coat the back with epoxy and a cloth like carbon fiber or fiberglass. Some kind of thin reinforcement. Then, fasten it down to a flat board and use a router sled to thin it down.