As my handle suggests I work at a hardware store, so for things that need durability I use the available options. Here's my quick easy solution for a spool holder. Currently it has space for 14 spools of any size spool I've run into. The basic version I put together in the first picture is two 6" corner braces, two 6" mending braces, a 1/4" x 12" threaded rod (it's actually a 10-24 in the picture, it was just the easiest to grab.) Two nuts and two wingnuts hold the rod to the braces and it could easily hold 3 or 4 spools. To expand it obviously a bit wider isn't bad but wider than the printer doesn't work well. You have to be careful with the outer ones but making sure you don't tangle filament with other spools is the important thing. Going upwards I used a 10" mending brace which just gives the largest spools enough room over the smaller ones (Dremel spools are only 7" in diameter and have a small diameter inside hole.) The double-decker version makes it a bit harder to change the bottom spools but as long as the wingnuts are tight it holds together. Also on the bottom mending brace "feet" I used rubber coated nuts called well nuts to keep it from scratching the table. Stick on rubber bumpers would work as well.