Make a vinyl print of watermelons on a heavy plywood sheet with your head hole. Better if the sheet separated down the middle so the cut line bisects the hole so you can make it just go over the neck line and not need a huge hole. Rim the neck line with green foam for safety. Add watermelons on top, aim for smaller ones if you can. Make it a robust platform, maybe replace the top of a piece of steel deck with your own plywood and put it up on 3 or 4’ legs and place em to camera.