My Camper is Moving Sideways in my P/U Bed

If you look closely at the picture you will see that I did mine in three pieces: one is 4 ft long and about 5 ft wide at the front, extending to the bed sides to make sure it doesn't shift sideways. The other two are left and right halves about 5 ft long aft. The aft pieces overhang the end of the bed by about 8" to ease the 'hard edge' at the end of the bed. That ends up taking 3 pieces of plywood (with a bunch left over) if you don't want more joints. I did it that way so it would lock into place and still each piece be small enough to carry. But there are plenty of variations on this theme. I only allowed about a half inch total sideways clearance (the Bigfoot has rounded corners which effectively adds a bit to that), and it helps as a guide to getting it lined up perfectly when you put the camper on. If you are traveling it is pretty easy to knock that together in a Home Depot parking lot with a battery powered sawsall. In finishing mine, I lined the rails on the sides with 1/4" polyethylene sheet material - but I don't think that was really necessary at all. The camper doesn't seem to rub much on the rails.

Like you, I liked the poly bed liner for its protection, starting again I would have kept it. In fact I may put it back in one day...