Commercial available ones were to wide for our use.......rear truck door on quad cab so dog could get up/in backseat.
Built one that was 14" wide outside to outside and 48" long
1----1" X 3" X 8' (cut in half)
1----1" X 2" X 8' (cut into 12" lengths)
2----1/4" paneling 12" X 44" pieces
Some carpet to cover paneling
Some 1 1/2" wood screws
Screw the 1X2x12 2" down from each end of 1X3 and then every foot
I made the bottom edges of 1X2s flush with bottom edges of 1X3s so that paneling pieces fit down into the 1X3s
Screw paneling down into 1X2 cross pieces
Cut one end of 1X3 at 45* angle on top edge (flat edge goes onto lip of truck door sil) cut other end at 45* angle on bottom edge so it sits flat on ground.
Cut carpet to fit side to side on panel and long enough to wrap around top/bottom 1X2 and secure into 1X2s with screws/washers
This was all done with wood I had laying around.
Plywood vs paneling