I have made some progress on modeling the battery holder, using OpenSCAD tool and a 3D printer to iterate the design a few times. There are a couple of critical dimensions on the BP-1A battery pack, notably the amount of latch tab “engagement” and the depth of the “step” that fits beneath the calculator housing towards the top of the cavity. But, I think I’m there, now I’ll upload the files to GitHub soon.
It seems that PLA is not the best material with which to make living hinge features. It’s quite brittle. The retention tabs have broken off from most of my prototypes. I suspect the best solution is to find someone to print it in ABS - or perhaps send to a service and get even better quality.
I also managed to enlarge the PCB dimensions a bit; now 30x50mm. This should help with the crowding I’m facing in layout, trying to avoid interference between SMD components and the LiPo battery pack.