I initially paid for EMS shipping for my order from Elecrow, but for some reason, customs/EMS Post rejected the package and sent it back to elecrow after a week. Elecrow were very helpful after I contacted them, and had it re-shipped via DHL. The package arrived within a couple of days, so from now on I'll spend the little bit extra for DHL over EMS.

I quickly tapped M3 threads into one of the back pieces, and did a dry fit of all the pieces. I quickly realised the holes on the front piece were not aligned properly. Turns out I'd misplaced them 1mm too high on my cad design / DXF file... oh well. I want to make the back piece much closer to the battery holder (now I actually have the physical holders I can confirm dimensions - the joy of buying random things from Aliexpress without part numbers or datasheets).

My idea to use pogo-pins for programming through some well placed holes in the case turned out well. Elecrow's laser cutting service has no issues cutting such tiny holes (approx 1.8mm if I remember correctly).