I use a nice cheap British Army issue dossbag with a center zip, yes they are bulky but that doesn't bother me! its a 4 season bag that cost me nothing (I liberated it when I left the Army! ) but you could probably get one for £30.
It all depends if your a lightweight camper or not.
woukd it shove into the bottom of a sabre 60-100? thats the max size i am looking at, ideally have some room left for the tarp too