Walmart has the Bell 510 Tote Metal Handlebar Baskets in some stores for $16.96 (mostly black now, but there may still be a few gray ones floating around):
https://goto.walmart.com/c/2586673/565706/9383?veh=aff&sourceid=imp_000011112222333344&u=https%3A%2F%2Fwww.walmart.com%2Fip%2FBell-Tote-510-Metal-Handlebar-Basket-Black%2F116398447%3Fathcpid%3D116398447%26athpgid%3DathenaItemPage%26athcgid%3Dnull%26athznid%3DPWVUB%26athieid%3Dv0%26athstid%3DCS004%26athguid%3D3e39a577-007-17396580cfb28d%26athancid%3Dnull%26athena%3Dtrue
Although intended to be used on handlebars, their big loops allow them to drop over the oversize tubes on Rad rear racks nicely and although you don't need to use the included mounting hardware, a couple of zip ties to keep them from bouncing off/around would be a good idea on all but the smoothest short trips (but it sort of of defeats their super easy on-offedness).
They aren't very large, just convenient for a few items.
Looks like the same thing is available on Amazon for $25+:
https://amzn.to/33d7d8k
Nothing will ever take the place of my beloved milk crates though, particularly the double longs, but these complement them nicely.
Great tip! Thanks for sharing.