There are a couple of options for out-of-the-box-compatible STANAGs. EB-Tech makes the ones that VT supplies with their rifles, and they're a bit heavy but work well. WA-spec mags are significantly wider than real-steel ones. I looked at it earlier and getting GHK internals into a real STANAG shell would be difficult. Probably not impossible, but probably not worth the work versus modifying the GHK shells.
After a lot of frustrating sanding and fitment I realized that the main culprit was that the GHK mags aren't bent perfectly, and they bow outward ever so slightly in the middle. Giving