My latest skirmish in the never-ending battle with filament spool encroachment. These racks provide excellent storage density, pretty much doubling what you can get vs a shelf of the same depth, and they look kinda cool, too.
The shelving was bought at Lowe's, it's their "Style Selection" brand of heavy-duty double-row rails and brackets. The brackets are formed with a hollow slot facing upwards, into which the tangs on the bottom of the printed parts fit. The pipe is standard 1/2-inch black iron plumbing pipe. I bought 72-inch lengths and cut them in half for 36-inch racks. Galvanized would be fine, too, but it's twice the price of black iron.
The rear pipe holder has an extended tang which backs up against the shelving rail. The front holder doesn't have this, but you could print spacers to go between the front and rear holders to prevent the front ones from sliding backwards if necessary; so far it doesn't seem urgently needed.
If you want to get really crazy you could probably slide short pieces of plastic tubing over the pipes so that the spools could rotate and you could feed your printers straight from the rack...
Keep in mind that fully loaded, each of these racks can weigh nearly 50lbs, so screw into studs if possible, and if not, use good sheetrock anchors, use them properly, and keep an eye on them.
UPDATE 12 May 2016: I added new "clip-on" versions of the pipe supports, because having the pipes accidentally lift up out of the supports while loaded with spools is a Very Bad Thing - don't ask me how I know. The pipes snap securely into the new design - colorFabb PLA/PHA's flexibility helps here, not sure if regular PLA will work.
UPDATE 7 JULY 2016 Tinkercad link in case anyone wants to customize it:
3/4" conduit is about 2.3mm larger in diameter than 1" iron pipe, so to convert this design to work with conduit you'll need to make the holes bigger and adjust the hole height in the support brackets for good snap-in action.
The pipe holders were printed in colorFabb PLA/PHA (for its outstanding strength) on the Raise N2, and the end caps were printed in Sainsmart TPU on a Zortrax M200 with Z-Temp and ZT-HE hot end.
You could print the caps in rigid plastic, but they might need to be redesigned for a good fit. If you cut the iron pipe like I did, you can end up with threads on one end but not the other - using flex filament allows enough stretch so that the caps will fit either.