The ones from rusmilitary.com have a plastic rod attached below the follower, inside the spring, so that the follower doesn't go down any further.
Tube shotguns use the same idea... a plastic dowel of some sort.
You may be able to "sacrifice" a follower and put a longer side on it w/ a length of sheet metal and maybe pop rivets or something like that.
OR you could sacrifice a mag body and drill a hole through it for a bolt or a rivet to go, physically blocking the follower that way. Would need some way to keep from interfering with the spring, however.
:dunno: what if you removed the floorplate from a 10 or 20 rd mag , drill a hole in the center, and attached a piece of threaded tubing with a screw to the floor plate to block the follower from going too far down ... might take some time to actually measure the whole thing and try to attach the threaded rod to the floor plate while trying to slide the whole thing back together , but at least it won't interfere with the spring movement:dunno: