I have the insulated Nashbar back you speak of. Mine looks like a big cooler
Thule actually makes a bag specifically for the rear rack you have and that MIGHT be the only option since I am unaware of how many holes or mounting options that rack has. I would also say is if you need something insulated, get a cooler a nice one. I have not seen too many great rack top bags with insulation that are actually good at keeping things really cool.
At this point I use a Stanley Food jar with usually liquid lunches but have put many many other items inside of it.