Here may be an idea worth considering.....On a recent Pre 66 110 build I replaced the box magazine with a 1/16" thick aluminum plate approx 1" x 3-7/8" long.....This plate slides right in the grooves that the original magazine was hooked in....A little trimming and filing on each end of the plates is required to match the radius of the receiver....I mocked up the plate with cardboard before I made the plate to test the idea, it worked great....Then I cut the plate from a chunk of aluminum angle that I had laying around with a hack saw....I then filed it for a perfect fit.....Works perfect and didn't cost me a dime.....Now a round or even an empty can be dropped in and always will feed with ease....In your case I would fill and cure most of the magazine well before final bedding...Use tape and plenty of release agent on the plate...
Good Luck,
Randy