Ikea's Hejne using the wider wooden soft pine shelving storage units has nearly perfect dimensions (~20" Deep(/Wide) x~31" Wide(/Deep) x ~5' Height; shelves are $14 USD for 2-pack of 30-3/8" x 18-1/2" with lag screw hardware, & $6 USD for 2-pack of posts, $1 for 2-pack of plastic "feet"), but it will need reinforcement to reduce wobble all around (2x4 wood studs, 1/2" C-channel or angled iron/aluminum, or wire braces, etc.) Pre-drilled shelve spacing is ~5" center to center, or just under 3-3/4" high between installed 1-1/4" square cross members on shelves. I posted a while back on this, something along the lines of "move over lack rack, hejne is here". It's stated shelving can handle ~100 lbs/shelve. reasonable for most mFlex, mATX, ITX sized systems and switches. Easily fits 12x, maybe eek 13x or 14x, 2u systems with a minor mod (drill holes) for one additional shelve at the very bottom just above plastic feet. Of course with every other shelve gone, 6x 4U systems should fit just fine with an ~1-3/4" of clearance, or 8-3/4" between shelves.
Borrowing the design and using a hardwood would also work just as well, if you have the tools and are so inclined. Good luck!