Took a 1/16" bit and drilled about 3/16" of the way into the retaining post on the sub-plate, just enough to let the screw start threading itself. Assembled the mag w/a different spring (see my other thread). Took a 5/16" long eyeglasses screw and put on 2 #2 and then 1 #4 flat steel washers and screwed it down. A little black paint to kill the glare and act as a temp. thread lock until I get some purple Lok-tite.

I wouldn't say it won't come off if whacked hard enough, but it's 10X more stable than before. Need to go test the spring now.

Without digging my Glock mags out, I never had a problem with Scherer products. The only thing I can figure is you may have an old magazine without the button in the middle? I think I put a Glock +2 bottom on an old mag, and there is slop in the fit on it too. I also put Glock flat floorplates on Scherer 33 rounders to make them 31s and they fit perfectly. If you gotta do all that work to keep them on, something is not right.

Ja, the regular Glock floorplate fits tight on this mag, while the Scherer could be bumped off with moderate thumb pressure (loaded).
The "tongue" should be a lot stiffer I think.

I was mainly bored and decided to tinker with it. It seems to have locked it on there pretty  good though.