Point taken that there are some viable options out there, so maybe this quest is more about price or availability than a brand new design.

But there are some tanks that cant fit the standard heng long style "4:1" boxes, only "3:1." For those tanks, what other options are available?

Also many options arent available from US sellers, and some of these "boutique" boxes are very specific for the models they work with. For example, I have an eto armor box which is phenomenal, but it is designed for Tamiya tanks and required quite a lot of fitting filing and drilling to get it working in my Torro KV2.

Is there something we can come up with that is the equivalent of 4:1 but fits in any heng long style tank? A design that is low profile and able to made with easily sourced (maybe even re-used) parts?