This remix of the single part tray extension by andreq is big enough to support 3 kilogram spools. I made it as large as I could for my Taz 6 build area, while keeping it a single part (plus an adapter for the lid). For smaller printers you will need to split this into multiple pieces.
The end result is about 4.5 mm clearance around a 3 kg spool. Not great for airflow, but it does seem to work.
I see about a 10 degree (fahrenheit) difference on the top of the spool verses the bottom during dehydration. You might want to flip the spool over periodically while drying.
PolyLite (PolyMaker) PLA
I used TinkerCad to do this remix. If anyone is interested, I'll publish the model publicly.
How to Use It
To use this, put the main tray extension in the dehydrator base, then put the 3 kg spool inside (trying to keep the same amount of clearance all around the spool). Place the lid adapter ring on top, then put the original dehydrator lid on top of that.
If you want, you can even put andreq's extension on the lid adapter ring and put smaller spools on top of the 3 kg spool. For grins I loaded up two of andreq's extensions and the result looked like a small rocket. I probably won't use it this way because I'd be concerned about decreased effectiveness with too many extensions.
So far I've used the single 3 kg adapter to dry a couple of rolls of PLA. I ran it at about 120 degrees for 8 hours, periodically flipping the spool to try and even the heat distribution. Seems to work so far.