Place a large tarp into the truck bed so that the bottom and sides of the bed are completely covered and the four ends of the tarp hang over both sides of the truck.
Tie a piece of rope to each corner of the tarp and connect those ropes to the truck's frame.
Tape loose sides of the tarp against the lining of the truck bed with duct tape.
Fill the truck bed three quarters full with water.
Insert your garden hose into the exhaust pipe and seal the pipe with tape so there are no leaks. Place the opposite end of your hose into the water.
Run the truck's engine to heat the water in your portable hot tub.
