If you know what gauge (thickness) this existing brass is , I'd suggest that you buy a roll of 'shim stock' , which usually goes up to .020 in thickness .
alaskacopper dot com sells rolls @ 6"x100" .
I've done business with them here in the Pacific NW & they've always done good by me (manufactured metal products have fixed costs , though) .
You might be able to buy some from/through a local machine shop too .
Shim stock is kinda old-school , but it's still available .
Happy hunting !