I made this filament rack after seeing a similar rack built by uCHobby (Filament Rack Adapter https://www.thingiverse.com/thing:2294603). I was looking for a more vertical rack than any I found online. Unable to find a a rack that would stack vertically, I designed one using Autodesk 123D. By adjusting your conduit lengths, this will create a rack as tall or wide as needed to fit your available space.

This rack uses 3/4 EMT galvanized steel conduit (a bit overkill as other designs used 1/2", but still cheap). The one pictured is 2.5 feet wide and the vertical support conduit pieces are 9" tall. Although 3/4 inch conduit is 23.4mm OD, I had to design the holes to be larger (24.0mm) for the fit I desired. Should your 3D printer's ability to reproduce correct sized holes is better than mine, you could always slightly scale down the print by a few tenths of a percent to achieve a firm fit. I've included a small test block with the same hole size as the rack brace so you can do a test fit before printing a whole brace.

Check out the spool holder I designed as an add-on feature: "Spool Holder for the Stackable Freestanding Filament Rack" (https://www.thingiverse.com/thing:2953103). I extended the top bars over my printer and added a spool holder to get the weight of the spool off the top of my printer and add multiple spools holders above the printer.