@Rongzhong Li Thanks Rz.

@Alex, you mentioned "getting at the motor current *somehow*". FWIW I've been mulling over using ACS712 sensors for similar purposes for a while. Haven't actually tested yet. Just thought I'd mention.

@Dobromir it’s a standard hex M2 12mm acrylic stand-off. It’s fixed to the bittle below by a 20mm stainless steel M2 bolt. I carefully drilled out the screw holes in bittle with a 2mm drill and passed the bolts right through bittle’s base. On top, a 10mm stainless steel M2 passes through the 3mm clear acrylic sheet which acts as a back cover, then an M2 acrylic nut acting as a spacer, then the RPi, before screwing into the same 12mm M2 standoff. All these parts are available on EBay.