you have the slug bolted in, and the magwell bolted to the slug, right?
I don't know how it could pull off, and filing shouldn't change the mounting angle, unless you filed off the back of the slug, and not the front.
I had to file the front of my heavy slug to get it to fit, but I also had to modify my basepads to seat the mags.
Stock basepads won't work, Taylor pads need filing. I assume Dawson 140mm pads will work with no mods.
Try taking the plastic insert out, and seating a mag that way.
If you don't want to reshape your mag extensions, you could always reshape the plastic insert and smooth it out again.
I'm still waiting on a white insert for glocks as well. Anyone heard anything on a release time?