X3
My experience with carrying things on the rear end have shown 2 general problem areas:
1. Dead carrying weight- reciever hitches on the rear of trailers generally don't handle dead weight that well. This typically is for 2 reasons, RV trailers aren't designed for it and there aren't any real good custom fit reciever hitches designed for that purpose. The only thing we recommend is a universal fit hitch called a Vari-hitch. It is an adjustable width hitch that must be welded to the frame. Even at that, we don't recommend any more than about 200 lbs of bikes or cargo. Another thing to consider will be the "G" forces exerted on the reciever from the weight of cargo as the rear of the trailer goes up & down with the bumps in the road. If you have 1,000lbs on the rear and hit a good bump in the road causing a 1.5 G-force, the weight exerted on that hitch system just went from 1,000lbs to 1,500lbs.
2. Side to side twisting- There isn't any real good way of preventing it. You can limit or reduce it, but whatever you stick in there, it's gonna twist back & forth.
Installing a reciever tube on each frame rail is not a bad way to go about it. The dead weight will be distributed between two points, and two points will pretty much eliminate the side sway.
I would like to suggest looking into a enclosed cargo trailer. It'll work with most any weld-on reciever hitch & ball mount on the rear of your trailer.
--Smaller trailers will hold 2,000 lbs easilly.
--Things won't fall off the back of it.
--It should stay dry inside.
--The doors are lockable providing secure storage.
--You can take the small trailer itself for day trips with the scooters inside.
Most states allow double trailers provided the 1st trailer is a 5th wheel.
A good reference for which states allow what is
HERE.
By all means stay away from those little reciver tubes that attach to the square rear bumper using square "U" bolts. It will likely twist & shread the rear bumper and cause you to loose whatever you were carrying back there.