I have a few different thicknesses of neapream (spelling) in my truck for stuff like that.
Silicone might work but that comes down to the surface on the back on the tub. If there is no room at the bottom you will not be able to get a seal with the silicone.
It might be time to cut out a bit of the wall on the back side (you should have a door of some sourt).
For the next bath just get out the duct tape and a Ziploc and seal it up. You can do it and join in on the redneck games.