If it doesn't fit inside (like above pic)you can lay two 2x4s across the bed spaced about 5 to 6 feet apart. You can use folded towels under the boards if you don't have a bed liner or top rail cover. Then we use ratchet straps and rope with truckers hitches tied in them to firmly tighten the tub down as it sits level on top of these two boards. Once again, place folded soft towels under any straps or rope where in will contact the trucks paint. Don't strap down so tight you fold down the tubs top rails.
1975 CJ-6 1983 CJ-8
1986 CJ-7 Laredo 1986 TJ-7 Trail Jeep
2003 Inca Gold TJ Rubicon