I built one about eight years ago.
I never looked very closely at the commercial ones, but wonder how you can make any turn at all with a fixed tray? With the front axle strapped tightly-down, you would be towing a two-axle trailer with no ability to swivel the front axle, unless the tongue swivels. The front suspension won't allow much strain left or right, just the tires and straps stretching....
Mine works fine. Only thing I would change is making the top of the kingpin lower, as I have caught the bottom lip of a minivan's bottom air dam on it ((no damage to lip, was plastic and just popped over). I used a blueprint I bought from a publisher.
The one I built is extra-heavy and I have pulled a WWII Dodge dump truck with it a couple of times with no tire or bearing overheating. Used Jeep Grand Cherokee front bearing assemblies, lug nut pattern same as common large trailer tire/rim combos.
For backing, I would suggest making a drop-in lock pin through the tire tray to the frame, although if you REALLY screw up, you could bend the frame.
I always just park where backing will not be necessary, although most semi double-trailer drivers are able to back up at least a yew yards.
Keep 'em rolling!