Thanks to @bbeaudoin for designing this spool holder. I printed this once, and made nearly zero modifications to it, and it fit my Dremel 3D20 perfectly.
The only hiccups I had were that the screws that came with the 3D20 that held the original spool holder in place do not work with this printed spool holder. (It's a weird thing: the hex screws, once removed, simply slide in and out of the pre-drilled hole in the frame. I can't figure out how they were screwed in originally.) I had to buy a longer and slightly thicker bolt to go into the hole, and that secured it perfectly.
Also, while the wingnut as printed fits, I found using it to secure the Hatchbox spool adapter on made it too tight to turn easily. It's been fine on the spool adapter without the wingnut thus far (one fast print and two long-ish prints).
Finally, as others have said, if you turn the model onto its back before printing, you can print all three pieces (holder, adapter(s), and wingnut) without any supports.