One idea for a backstop is to make a small box with two or three layers of Coreflute lining the front face. Coreflute is partly self-healing when projectiles go through. You'd fill the box with sand or similar which would prevent any voids for pellets to get through, something that clay clearly isn't good at. I haven't tried it but I'd guess 50mm of sand would stop any pellets, even from a PCP.
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