Maybe you could luck up on a thick steel plate like they use to cover holes in the road during repair work. Maybe a road construction company or the city might have a small piece or some scrap they would sell you. Lay it in the bed and leave it.
Chuck 1950 Chevy 1/2 ton (all original) 1951 Chevy 1/2 ton (future streetrod) 1941 Chevy coupe 1938 Chevy coupe streetrod