To add a "floor"/supports for boxes fairly economically, you could purchase a length of metal rod from Home Depot sufficient to add 3 horizontal supports that would run from one side of the trailer to the other. Drill holes through the existing aluminum frame and secure with friction washers. 3/8" rods should provide 50-90lbs of support if the load is spread across all 3 rods. Additional weight capacity/support could be obtained by purchasing a plastic tarp from Wally World and gluing/sewing it around the existing frame members. I happened to get one on sale/clearance this morning for $5 @ 6' x 8' size due to the package being damaged (someone had poked a hole in it to feel the plastic tarp material).
-----------
You could make rails from 1/8" metal rods obtainable from Home Depot. Bending these is easily accomplished using a vise and pliers or even just pliers if you're careful/strong. These can be made to your desired height by measuring and cutting to length with a hacksaw/metal cutting blade on your skillsaw. Remember to measure from the bottom of one aluminum frame tube up to the desired height, across horizontally as far as desired, and then down and through to the bottom of the aluminum frame tube plus one inch (for slop/fine-tuning adjustment later. Cost should be under $25 I would think. The advantage is it's fairly strong and you can make multiple height rail sets for different uses - or even just front and rear rails while using bungee cords for side rails. The disadvantage is the rods will be heavier than aluminum tubing, but not exorbitantly so.
To mount the rails, you have a choice of 2 methods.
First method: Make wooden plugs for the existing holes. Drill a hole down the center of the plugs which is the diameter of the aluminum rods. To prevent the plugs from dropping through the hole entirely, shape it like a presta-to-schaeder tube valve adapter - smaller diameter section goes into the existing hole in the aluminum frame tubing.
Second method: Drill holes in the frame itself then use 2 friction washers on the rods themselves placed above and below the holes to stop any vertical movement of the rods. To add structure to the rails, you could bungee, cable tie, or even use electrical tape to join adjacent rods near their tops to maintain near 90 degree angles.
Bending aluminum tubing is not difficult in and of itself, but if done incorrectly, can result in collapsed tubing.
AFAIK, aluminum tubing cannot be unbent absolutely straight without heat/weakening it.
-------------------
Another alternative would be using pvc tubing rather than aluminum tubing. 90 degree joints can be purchased rather than fashioned by bending. The total cost might render this the most expensive solution, however. Weight wise, this should be less than the metal rods but more than the aluminum tubing.