My initial doodles suggested an end cap with an opening for the Arduino’s USB port and something for the power cable from the Bafang controller:
Common sense finally broke out and I made a simple disk cover held in place with an M3 screw:
Unfortunately, I cut the PVC shell flush with the USB port, which meant the cap couldn’t have a little shoulder to stabilize it on the shell. Maybe next time?
Machining the disk required using the scrap of aluminum rod left over from the heatsink as a fixture with a piece of sandpaper stuck to the front surface:
The live center presses the bandsawed + disk sanded cap against the sandpaper, providing barely enough traction for sissy cuts reducing the disk to the proper diameter:
It actually worked pretty well, although next time I’ll skip the sandpaper, affix the disk directly to the double sided duct tape, and be done with it.
Line up the center punch dimple and drill a hole for the M3 screw:
The power cable port turned into a little slot bandsawed into the edge of the disk with the sharp edges filed off.
Basically, the thing needs some road testing before I build one for real …
The Smell of Molten Projects in the Morning 
A trick I learned from Clickspring on Youtube is to machine concentric grooves on the work holder chucked in the lathe. Then glue the workpiece to the grooves with crazy glue. The joint is ahem crazy strong. A little heat afterwards and the workpiece breaks right off.
Ever since I deliberately glued my fingers together to see how well cyanoacrylate worked as a skin adhesive, I’ve been reluctant to get near the stuff. It was a long time ago in a universe far away, but … lesson (over)learned!
As long as you don’t glue your eyelids to your eyes as my friend once almost did…
And it’s worth noting that hot superglue supposedly releases some nasty stuff you’re not to breathe in.
But if you get a sudden urge to try your hand at it, then sell it in applicators resembling a small two finned rocket that makes it easier to handle.