I made a 2nd deck, just like you need. It was real simple and cheap
Once I made up my mind how I wanted it, and where I wanted the brackets to be
I asked my old boss to sell me exhaust pipe, I bent the pipes with the bender at 90 degrees on each end, I built 2 of them along the middle of the trailer (they looked just like a stairway rail in a swimming pool)
Then I welded a couple of pipes across each rail for extra support, and there she was! The whole assembly begging for installation
The 2nd step was to find more junk/left over pieces of exhaust pipe to build the receiving brackets. I cut 4 pieces of pipe (same lenghth) I flared them up to fit the legs of the deck, then I welded 2 on the back of the trailer, and then 2 on the front.
When it was all done I could pick up the whole assembly solo, and stab it in the holes, and be done with it, didn’t even need fasteners, as the waight of the kart would push it down. I could put 2 karts on top side ways , no problem.
Then come home, unload the karts, remove the rack ( 2 minute job) Remove the 2 bolts and fold it up, put it away, and no one would know what just happened. ;)
Sorry, no pictures here, just try to picture in your mind. I will work
It was a few years ago, and I only paid my boss $40 bucks for the pipes.
P.S. I never had to worry about having extra hubs or buddy, as when I put it together I repacked the bearings by hand with real good bearing grease, plus the hubs had a lube fitting for a grease gun, never had a failure with them, and I used to carry heavy loads of building materials, appliances, furniture when we moved, etc.
Anyhow, loved that little trailer. Hope this helps you. Fast Freddy. ;)