Oh kay......
What the he!! is DUCK-BILL-LESS? 
I have another definition Matt.
(think Forrest Gump)
Duck Bill, less you want to get your head knocked off!

OK so I know it was a stretch at best, so shoot me

On a serious note, Marcus I have a couple of the old Tapco 5 rnd mags that I use for hunting. They are hollow on the bottom side of the duckbill. Have you tried these with your adapter or do you have a solution? The only things I can think of off the top of my head are to fill in the cavity with black epoxy or cut down a Tapco 20rnder to a 5 or 10 rnd mag(was going to type clip, but didn't want to get blasted, lol).