this is an insert that goes into a modified usgi magazine. All you really need to do is cut the feed lips off flush with the slot at the top of the magazine.

Blackdog machine sells springs for 25 round magazines that work in these.

once you insert the conversion kit into the top of the modified magazine body you can either hot glue them in place or let friction hold them in place.

these work with OK industries and colt magazines, I haven't tried any other manufacturers yet.

update: i just realized i uploaded the wrong files the first time, they should be good now.