I'm not sure about brass.030" having much part in reducing flux strength. I say this because there are millions of magnetic guitar pickups out there that have brass covers that thick. Some nickel plated,chrome or gold plated. Many have no holes at all to let the magnet adjustment screws stick up through. The Gibson Johnny Smith pickup has no openings. The Fender Telecaster bridge pickup has none at all.
Do you know that everything,even water IS magnetic if you have a strong enough magnet? I saw a program where this scientist has a mega strong electric magnet with a hole through the center. He even suspends small frogs magnetically in the hole!!
It seems that gravity,magnetism and (I think)light,or it could be electricity are made of the same stuff. They haven't figured out what that stuff is,yet.
I make lots of brass or silver parts for jewelry casting models. I just SUPER GLUE the parts down. For Pete's sake, DON't use some rubbery stuff like contact cement!!! To get parts loose,a little heat will melt the super glue. Then an acetone soak will get the rest off. I clean my gluing surface with a wood working chisel. Super glue is RIGID,which is what you need.