My setup is probably more cost and effort than you want to go to, but I welded some tabs onto my rack basket, used to mount Yakima bars. The customer service people at REI can order individual Yakima replacement parts, so I had them order a set of the aluminum loops and plastic saddles that are normally part of the standard 1A raingutter towers. These attach to the tabs with a single bolt, and then I can use any of the Yakima attachments. Not visible in the following picture is a third set of tabs in the middle of my rack, so I could install an additional bar for carrying shorter objects, or attachments such as a bike rack:
For another view of what I was using prior to building the basket, see this:
That's just a simple piece of angle adapting the above mentioned parts to a Kargomaster Congo Cage. They have changed the design of the Cage so that exact arrangement won't work with the current units, but it gives you an idea of what can be done.