The coil spring I bought was way overkill so I decided to buy a cheap mountain bike at a pawn shop and use the coil from that. I also added two gas shocks. Not sure if it will all work but it's worth a try.
My bed is removable in case I decide to change it. Two pieces of angle create a channel that slips over the "backbone" tube.
Eddie, the upper bed is 45.25"x25".
Overall length from vertical (side to side) pivot to rear of tire is 97".
Axle of bike to axle of trailer is 101".