Wrench, Hahn makes an adaptor to use sten mags, and they also sell one for grease gun mags. Don't know if the grease gun adaptor is a drop in or not. I'm leaning towards making an adaptor like the hahn. There are two styles of these. One thats basically a block like the Olympic mag adaptors, and another one that is ar15 mag profile and the mag sets about in the middle of the mag well. I'm going to make the block type.
I got some more work done on the 9mm build. I have been able to get it to fire semi-auto using the sten mag with a block of plastic jammed behind the mag to hold it in place temporarily! Since it did run, I went ahead and profiled the gas block so that it would fit under a handguard. The front sight is pressed on and held with a set screw at this point. Once I'm ready to parkerize it I'll drill the front sight pin holes. When I get time after my mag block is done I need to make a flash hider for it, but in the mean time the one thats on there will protect the barrel threads. Here's some pics.