Right, finally picked this project back up!
Spent a few hours modelling the trigger box so I could print one out, then made a few different versions in an attempt to get the system to interface with standard black dog M4 magazines but there just isn’t enough room for the mag to work also leaving the ejector in place (ejector not extractor) and the whole thing being strong enough.
It looked like 15-22 mags might work, but they sit a bit too far into the chassis to work
So I think the way I’m going to go is to design and print an insert that slots into the airsoft metal magazine shell, which is slightly shorter than a standard mag, and then uses the standard 597 magazine design.
I might still use the 3d printed trigger box because I can make it slightly shorter which means the whole thing sits together nicer in the chassis