Hi,You can make one , piece of thick ply, then screw two strips either side,I've done ot and I ain't no carpenterIf yo want pic let me knowMr B
Bit more complicated I suppose but if you make up a "ladder frame" with 2" x 1" you can use a thinner ply making it a bit more lightweight. I needed about 5' long and 18" wide. Face with descent marine grade ply 1/2" and screw with counter sinking screws. Take a trip to you local diy and find some aluminium angle 1" x 1" and screw it round the edge. Finishes it off nicely and prevent the edges chipping. I bolted a descent piece of angle iron in front of the rear door catch and bolted a 90 degree plate onto the ramp and just hook it on.