I have one that looks pretty similar. If you undo three pin clamps the top rail folds down to one side, then both sides fold flat onto the bed.
I'd look for a light piece of marine-grade plywood to go on top of this. Fit a square of wooden battens around the edge and fit some hose clamps over the rails from below, and screw into battens.
Optionally fit some low sides to help hold stuff on the tray, but this will increase windage.
Or look for a plastic baker's bread tray instead, the sort loaves are delivered to the shop.
Its your call whether you want the sides to stay be up and slot the cargo tray between the uprights. This would give somewhere to mount lights and reflectors and maybe even a chopper flag. If you remove the uprights completely, then the frame will be weak and the tray will be required to supply bracing strength.
Carry at least one spare 20" tube at all times, build this into the plans.
My trailer has a third wheel and a pram handle too, so it becomes a giant kids pushchair. If I was shopping, I could push it around the supermarket instead of a trolley.
That's the same kind of trailer holding two landrover tyres and rims. Total weight there was 68 kilos, and that was on the upper side of comfortable. I managed to roll this trailer at 15-18 km/h on a roundabout, so it was too top-heavy.
Note the hitch has been upgraded. Originally it came with a truely-awful chainstay clamp that would undo over time even when empty. Now its got a proper towball and cup arrangement, and works much better.