I shimmed mine with plastic shim, worked ok, can get a assortment on e bay. I finally straighten the barrel and fell better about it now, not hat hard to do.
I've used 2 sided automotive tape, but the gun I was using it on was a harsh springer and ended up rolling the tape out the back of the scope mount. I would suggest sandpaper. It will bit in pretty good. It may scrape your scope up depending on the gun's kick, but that's what ultimately worked before I got a droop mount.