mysquishy is correct. I did similar fix to my USA mags, although I simply used a screwdriver to pry out the upper lip on the mag, rather than drilling holes from the opposite side. No problems with my modified USA mags.
new guy here. I had similar problems with USA-made mags either not staying in or releasing with only slight pressure on mag release, or simply by wiggling the mag in the well. I solved it by using a screwdriver to pry outward the upper lip of the mag catch notch on the magazine itself...