More progress - no progress!
I’ve got the second floor in the dehydrator - I have kept the standard trays to leave a circulation gap between the bottom, the two spools of filament and the lid.
I tested it for 90 minutes with two spools of filament - in the bottom a half spool of Prusament which has well ventilated spool sides, and a generic solid sided spool of PLA on the top.
I set it flat out (70° on the dial) and it was consistently 51° everywhere I measured. The bottom of the bottoms spool hit 57 which is getting a bit dicy for PLA, but there’s a dial for that and I was exploring!
My conclusions are that the extra height actually helps temperature, but slowing down circulation. Every thing I measured other than the surface of the bottom spool was around 50-54° - the outer surfaces of the spools, the outer surface of the filament and the inner core of the spools.
The sad reality is that this is certainly good enough for drying PLA and TPU - I think it would be “good enough” for PETG, and perhaps as a pre-drying gadget for more difficult filaments.
It certainly has a much more even circulation than the Eibos, but I’m still working on that.
Hold my beer…