I have a bunch of fairly heavy aluminum 90 degree angle-iron pieces scrounged up long ago. They are about 2" by 3/16" flats by 30" long, and easy to drill holes anywhere and attach with the usual 10-32 screws into the side members of almost any rack. Usually it's enough to just support the side rails of the gear firmly, from front to back. I put the heaviest items on the rails, and stack lighter gear on them, so only a few big items are actually mounted, and serve as shelves for the rest. It's easier to reconfigure too, unless you need to relocate the big stuff.
So, you don't really need a shelf pan, but be sure the gear can't move too far side to side and fall through. The wide enough angle stock mostly assures this. I drop in wooden shims on the sides if there's any doubt. Also be sure the rack is stout enough to not bow open - sometimes you need cross pieces here and there to hold the sides together
Ed
