wanted a standalone filament rack that was easily expandable and that took a small surface area to print so that I could maximize the number of pieces that I printed at once. The result is a rack that uses ½in PVC for both the vertical and horizontal bars. The rack can be built with as many layers as you want. Using just the top/bottom pieces will get you 2 layers, but the middle pieces can get you as many additional layers as you want. 

The end pieces do not have holes that go all the way through for the horizontal bars, but there are also pieces that do have through holes in the event that the horizontal bars are too long and you want extra support. These are labeled as the center pieces. 

I have added 5mm holes at all the connection points so that holes can be drilled and M5 bolts can be used to hold everything together, however I don't think that it is necessary since the PVC fits tightly. The 5mm holes for the horizontal bars are inside the holes for the vertical bars (this keeps them hidden), so if you decide to use bolts, the horizontal bars need to be bolted in before the vertical bars are attached.

I also have some extension feet for the bottom in the event that you want to add more stability. The feet fit around the horizontal bars on the bottom and each foot attaches with 2 m5 X10mm bolt. The feet can be put on afterwards if you think they are needed. There is a version of the Top/Bottom pieces with holes for the bolts and a version without. There are no feet for the center pieces. I initially didn't use feet but decided to add them after the fact. The bottom piece with the feet added is just over the size of my print bed, hence why they are bolt on.

I printed with 4 walls and 5% infill. I was able to print all 4 top/bottom pieces at once (with the vertical holes facing up). I printed 2 of the middle pieces at once and laid them down on their side with the horizontal holes facing up. The entire rack was printed in 3 prints, with the top/bottom pieces taking around 21 hours and the middle pieces taking around 16 hours. 

For PVC lengths, the vertical bars I used are 10in sections and the horizontal bars are 19in sections. The 1ft sections seem to allow for enough room to slide the spools in and out easily and the 19in sections allow each layer to hold 6 rolls of filament but still have enough room for the middle support if I want to add it later. I used around 25ft of PVC which I was able to pick up for around $13. ½in PVC pipe or PVC conduit should work (I used pipe since it was slightly cheaper).