I welded a length of 2" ID square tubing to the bottom of my frame on my last TT for 18yrs and now have the same on my 5er. With a receiver on each side you eliminate a lot of movement but you have to custom make everything. I have a 18" wide expanded metal tray with a 1" square tubing rail across the back. I used to stand on the 1" tubing to wash the rear top of the TT. I have a trailer ball that fits mine when not using the tray and I want to take the fishing boat along.
You have to make sure every thing is in alignment or you'll never get any accessories mounted back there. Don't forget the center the hole for the pin.