Norther Tool has a folding, 4x8, aluminum trailer kit. It will run you about $800, and take a day to assemble. Aluminum doesn't rust, its suitable for leaving outdoors. It also weighs a fair bit less than steel.
Now, the folding hinge design is pretty crappy. You are NEVER going to want to fold it up. Also, it has no bed - so first thing you are going to do is throw away half the bolts, replace them with beefier bolts and nuts with nylon inserts, plus drop the cash on a piece of 1/2" or 3/4" exterior grade plywood. Not the grade 2 stuff. Drill that, set your bolts thru it and into the frame. Now the frame is straight, true, and stiff.
It has stake pockets on the sides, that's how you will frame out the rest of your build. Also, the ground wires for the trailer lights are pretty crappy - aluminum doesn't carry current well, once it begins to oxidize. Consider running a ground wire from the trailer harness, rather than grounding from the frame. Finally, the trailer light brackets themselves are crap - you will get a season out of them, maybe two. A $20 investment in better light brackets is in the future.
Its light enough I can move it empty, one-handed, at 50 years of age up and down "hills". Even with a bunch of 2x4 framing and 4' wire sides, I can move it one handed with a little effort up and down hills, and I'm not particularly strong. Finished weight on mine is probably in the 300# range, it was 200# shipped.
Drag it next to the house, run an extension cord to your heat source for brooding, and you are done. Clean it out with a push broom. Oh 1 3/4" trailer ball. Having something heavy to attach it to when not in use is a WISE plan.
They have a non folding, 5x8 design too. Only a little more expensive. You will, of course, need more lumber for the floor, and it weighs a bit more (plus the extra lumber). Looking back, because I use mine to haul small lots of construction materials, that's the one I should have bought - but I tow mine with a Buick sedan - not much tow capacity, weight was a premium consideration.