Your grammar is just fine. (It's better than some of the native English speakers here, in fact!)

A shim of the proper thickness should do the trick. If you've got a properly-sized shim and parts are still slipping, using something like carbon assembly paste to add friction isn't unreasonable. I'd be inclined to measure parts and inspect closely as well, since properly sized parts (including properly fitting shims) shouldn't be prone to slippage when the clamp is tight.