I want to build a small rack on top of a rear hitch receiver cargo carrier, to hold an empty sewer tote. The cargo carrier has a mesh grate deck. See pictures

The angle flanges look like a good foot for my rack, as I would like to be able to fold the rack to fold the cargo carrier.

https://makerpipe.com/collections/modular-pipe-fittings/products/adjustable-angle-flange

What could I use on the underside of the metal grate to bolt the angle flanges to?