Use a 5/8 x 1 1/4 x 1/8 piece of masonite, rough side toward track. File or sand a taper on each end to prevent snagging crossings or turnout rails.
Drill snug fitting holes for nails on centerline 1/4" from each end. We used a 1/16 bit for 3d finishing nails.
Remove the boxcar floor and drill loose fitting clearance holes for the nails in the car floor. You can use a masonite pad for a drill template. We made a template of clear plastic (Polycarbonate) the same size as the pads which made it easy t o center the holes on the floor.
Push nails through masonite from rough side. Use a small dab of 5 minute epoxy or gap filling ACC to glue the nail on the smooth side of the pad.
When glue is set, clean any excess glue from nails so pad slides freely in holes. Push nails through holes in floor and slip a 1/4" long piece of 0.160" diameter screening spline on each nail so that the pad hangs low enough to drag on the track. Then cut the nails to clear the car roof. Make sure nail heads are flush with bottom of pad. File flush if necessary.
Insert floor back into car body. If you like, use a felt tip marker to blacken the sides of the pad.