the best setup I've seen is a steel plate cut/welded to fit inside the box, and bolted through the floor.
oilfield service trucks used to have these, 1/4 diamond plate for 275# or 1/2" diamond plate for 550#
never in the way, and easily removable with a lift
edit to add: my ideal setup would be a 3/8" diamond plate, not widened to go around the wheel humps, so you could slide it out. Add on top a few sand bags/kitty litter boxes for helping others for traction.
That should get you about 450#. You can move the sand/litter to front/back as needed.