You are already on the right path..... study what others have done. A golf cart is pretty light so your bed will not have to be to heavily built. Make sure that any frame members will not touch with the air bags completely deflated. I would avoid putting a frame member between the wheels like the picture shows. Instead use a piece of steel plate in this area like is common on gooseneck trailers. Be sure to include tie down points for your cart and a place to secure the loading ramps.
1989 FLD120 with 2000 NRC conversion