It sounds like the KWA may suffer from the same problem that my CYMA .030 does. They are actually the width of a .45 style Glock. That being said, I purchased the Serpa that is for Glock 20/21/37. My .030 fits in it beautifully. You can literally start the tip of the gun feeding into the serpa, and drop the gun, and it will fall fully into place. It does not rattle around at all and draws like butter when you push the release.
Are you in either the Madison Heights or Livonia area? If so, you are welcome to try my serpa for fit.
Edit: Nevermind, I was using the app on my phone when I made this post, which doesn't display your location. I just checked on the PC and noticed that you aren't even in the same state!
Definitely try to find a store that you can try your ATP for fit before buying another G17 style holster, I suspect that none of them will fit. I know it seems weird to bring an airsoft gun to a real steel store, but I've never had anyone at a gun shop take me any less serrious about it.
I have Safariland holsters for my real steal Glock 21 and M9. The M9 one is the holster I used in Afghanistan. I love both of them. They are miles and miles better than any of the Serpas I have used. I would recommend saving up for a bit and getting one. Just need to figure out if you need a glock 17 or a glock 21 model is sounds like.