After some searching I found a budget version for a tail bag. The Giant Loop and Wolfman bags are the obvious top shelf choices for this, but I wanted to spend about half what the premium bags go for.
The medium size version of the Held Iconic Evo bag fits like a glove on the PM Rack. I haven’t yet tested how much stuff I can fit in the bag, but I will report back once I have.
The bag has two straps with buckles at both ends. I suppose the straps are designed to go under a bikes seat, but work perfectly with racks too. Obviously. The bag is rigid enough to hold it’s shape even when empty (which is something I wanted).