I havent posted here on this, but I just usually drill the handle material and epoxy them in before finishing. I take a dremel tool, and with a cutting blade cris cross the back of the cartridge head that I have cut off in a grid pattern so that gives the epoxy something to bite into, make a mechanical lock, and clean them first with acetone to remove any oil, etc. for better bonding, never had any come loose. Hope this helps.