I'm sure that's not the intention in your case, but open center car trailers like the one below are quite popular here because it saves weight and allows oil change and other maintenance under the car. There are trailers with less cross-members, which suit even better for on-field grease-pit.
It is a clever idea to store the ramps in the middle!

I will be interested to see what kind of tiedown points you are going to make.
Regarding the safety, why would you need to walk on the car trailer?