I don't have an Axiom, but have a Topeak trunk bag that I just love. I had to buy the rack to go with it, but it was worth it (easy push button on and off- no velcro straps). they have ones that expand a whole lot more than mine. It has bungee straps on the top, and the top expands. Mine doesn't have the panniers that fold down on the sides, but they make them that do. I LOVE this bag. Don't know if you want to buy a new rack, tho, or not.
This is mine:
http://topeak.com/products/Bags/RXTrunkBagEX
If you don't HAVE to have a pop up top bag, I have an REI commuter rack trunk. It has drop down pannier sides and a rain cover (but no pop up top). It's in my pile to put it on eBay, but if you're interested let me know.
It's like this:
http://www.rei.com/product/780461