Steve, why are wood racks not ideal? Instead of finding something that your specific sized tanks can fit on...building your own out of 2x4 gives you the option to make it any size you want.
I have a bunch of these I want to use in the future for racks that I would just bolt together. I got them for free as I would never pay that price. https://www.uline.ca/Product/Detail/...B&gclsrc=aw.ds
...Ralph