that's a toughy!
if you really do want to mount it behind you, use a length of 3" pvc to put the 1# tank in and either mount the pvc to the tub or to some thing else like a length of wood that can come out if you drill it into the plastic sled with wing nuts maybe.
or make a full sized plywood base for the sled so you can run a bolt up and into a pvc cap/cover and then drop your 1# into that. some thing in this line should get you going!
i use mine in front of me so i can get the heat to where i want it and that's my face and hands after being out in the cold wind and want to warm up fast.