I got an EMG Salient Arms Hi-Capa variant Co2 GBB airsoft pistol in an Evike Box of Awesomeness. This model has a heavy metal base plate for the magazine, and a spring loaded latch that holds it on. It also has a somewhat sensitive mag release, so the day I got it, I bumped the mag release, the mag fell, and snapped the tiny little latch. This model is my solution! I have tested it out and it works pretty good, but will probably break as well, so I keep a bunch in my bag of airsoft gear, along with some spare 3D printed base plates.
I print it at 100% infill and .1mm resolution (with supports), to try to get as much strength into it as possible. I find it easier to use if you compress the spring up, slide the latch into the magazine groove, then let the spring seat on it otherwise sometimes it tries to go in crooked if the spring is attached first.