Amazing build Mike. Love the attention to detail. I really like the torsion axle frame hidden in the body. I’d like see a little more detail on how the tongue ties in to the cross member. Do those diagonal rails from the tongue extend to the cross member at the axles or do they mount to the plywood body?
I am away from home until July, so a bit limited in my reply. But I can say that the diagonal rails do not tie into cross member. The ends are welded to angle sections (at the base of the triangle) that are drilled and then bolted through the front of my storage box (3/4 inch ply) which is drilled oversize and filled with hardened epoxy and re-drilled. Inside the box are matching 1/4 inch steel plates to accept bolts and sandwich the ply.
The tongue piece has 4×6 inch plates welded ate the far end, top and bottom. They slip over and under cross member and bolt through it and continue through floor and matching steel plated underneath.
When I get back I can post some sketches and photos.
Lastly a disclaimer- I am no engineer and this may or may not work long term and / or be safe. Buying a manufactured trailer is the safer way to go.