Walter made two nice extensions for a standard food dehydrator / -dryer to convert it into a small filament dryer / dry oven, as can be found here: https://www.thingiverse.com/thing:2853136
However.... my Prusa MK3's bed is not large enough to print these in one piece. Hence I cut the 100 mm extension up in 4 pieces that you can glue together.
Walter has a small Westinghouse food dehydrator and I have a EUR 30,99 "VidaXL" private label and it fits perfectly (approx diameter = 24.5 mm). Maybe there are more 'cheap' food dryers have the same dimensions.
This will allow you to dry 1 Kg spools.
Suggest to set 3 perimeters and zero infill. Needs just a little support (set support generation such that only the overhang of the overlapping piece gets support - approx 8mm high only).