I had this exact same problem in my Nexstar. There's foam already on the cutout of the former/bulkhead which the tank fits through. So packing foam around your tank will make it such that you won't be able to get the tank to slide through the former and into the firewall.

I took a couple pieces of balsa and glued it to the fuse sides just behind where the back of the tank sits. Then I screwed two small cup hooks into it and threaded two #64 rubber bands behind the tank to hold it in place.