Bonus round: I ended up with extra cured carbon from the first failed ribbing positive, felt sick about throwing it away, so used it and some other parts to build an amplifier, which formed the base of a tv/bluetooth/multiple audio signal stereo system I hacked together for the shop.
"100W + 100W ...Ass..." I kid you not, this is what it says.
The little amp board wants 20-something to 32v input. Turn the pots on this variable voltage supply until it says 32.
Worked out the barrel plug for power and a 3.5 mm audio jack.
More of that perforated stainless from the captive nut plates repurposed into a vented lid.