There are few options for aftermarket box mags out on the market, and all approach the problem from slightly different perspective.
My approach is not only as a player, but also as someone who has spent time in the aviation maintenance field and now as an engineer. As such, I've baked into my designs a couple of key concepts.
From the fabrication perspective, I have a design-for-printing mindset such the shapes and angle minimize or enitrely eliminate support material, saving print time, material and post-processing time. Additionally, minimizing the number of parts helps reduce costs and assembly time. Similarly, commonality of parts among the various full kits and inserts reduces the logistics.
From the design perspective, I had a goal of keeping everything as simple as possible. As such, the feed mechanism is removeable for ease of maintenance or replacement (as required) and every single box mag or insert uses the same feed mechanim and most use the same mount system. Most of the hardware uses metric socket head screws to stay consistent with the common sizes found on most airsoft replicas. Installation of insert kits requires minimal tooling and effort.