This easy to assemble shelf uses brackets from Home Depot and metal conduit tubing. The part itself is reversible and can be underslung.
2 Home Depot angle brackets: 7.25 in. x 7.25 in. White Over Under Shelf Bracket EB-0051-8WT.. $2.29 each
1 10' length 1/2" EMT Conduit.. $2.34
4 1/2"-5/8" Screws and nuts, 8/32 or 10/32
White spray paint
I also included a label ring.
Print two bracket supports. I like 40% fill for these.
Cut EMT to length with a tube cutter or hacksaw. 42" is a comfortable stretch that shouldn't need a center support. The screws are for registration. Rivets or some other fasteners could also be used.