There is an epoxy putty which is real strong but can't take too much heat that doesn't need close fitting parts but is a little tricky to to get 'invisible' but being a putty it can be worked with wetted fingers to a fairly nice finish.

I must say though that seeing as the gun came with a scope base, why not fit a scope and enjoy? I would glue the base down so it won't move. The trick is to lightly tighten the screws then once the epoxy has set, to tighten them fully. My mini-14 got a scope mount that required epoxy to mate the two surfaces and that was the instructions. It worked! The base only had one front and one rear screw. It never moved in the twenty years I had that rifle. Mind you, I did use a high strength epoxy. It wasn't JB Weld but I have used that and it is good stuff.