You usually put two 1kg spool inside PrintDry Pro. I don't, I put four 1kg spool and dry them together. The black rubber prevents you from doing it, yet without the black rubber, your spool is too low and overheat from the hot air. So here is a spool holder that allows me to get four spools in the center. Diameter is designed for prusament sized spool, but expect a slim spool.

Print with abs/asa for >=75C drying. You can also print with other material if you know it can handle the temperature you are using. Assembly by inserting the black hard plastic ring inside the given spool holder, you will want 4 ring for each overload spool holder. Exactly using up all 8 black rings PrintDry give you.