* I first thought of building a platform for a tent, on top of a small, derelict frame I had sitting in my yard for many years, but changed my mind. It wouldn't have offered the advantages of a "hardtop tent" (as I sometimes described it to the wife, as I proposed an upgrade from her Coleman tent she would use), and though it could be easily towed, and used as a light utility trailer (very light load capacity), it just wasn't in the cards.
from another, earlier thread working on it wrote:- If you're OK with a tent, and not a hard-sided plywood (or foam, or "tin", sides and top), then why not just buy a normal tent (i.e. Coleman, Kelty, or even Ozark Trails), and build a fold-up platform on a trailer base, and pitch the tent atop it. When I first contemplated using my old derelict trailer frame (50" x 60", of unknown capacity or purpose), it was as a platform for my wife's Coleman tent. She would go camping with her Bigfoot organization, and would return with the tent full of mud, and all her clothes wet and muddy. I proposed that she should use the trailer base as a raised platform, to keep her out of the muck and mire (she obviously hadn't learned to pitch her tent on the upside of a hill). She said she might use it, but not if it was hard to assemble on-site.
- So, months later, our discussions trended towards my sketchy ideas of a "hardtop tent" (i.e., a "teardrop"-type of small enclosed trailer), as better and more comfortable camp quarters than a canvas or nylon tent. I had long planned to try camping again, after 30+ years of not doing any, so my ideas and needs soon overshadowed my initial plans, of building a simple folding base for a tent for her to use. I ended up with a "hardtop tent" alright, and it was too overgrown and overstuffed with my items, that she lost interest in it, and it became 100% mine. P.S. she also didn't like the confined feeling of the enclosure, made of 3/4" plywood with steel joinery, and preferred the "openness" of a leaky, drafty, insecure tent (Bigfoot might have trouble getting into my trailer, but not a tent). Her loss, my gain...she quit camping just before the trailer was finished anyway, so she probably wouldn't have used it under any conditions).
* The base trailer frame I was using needed various improvements to meet my minimal standards anyway, such as a lengthening it to 8' (you want 9'), with a lot of tubular steel (not needed for a simple 9'x 3.5' platform, though some extended perimeter framing would stiffen it greatly), and going from 8" tires (which I wouldn't have trusted at highway speeds), I increased their size to 14" (with commensurately larger fenders), and installed a longer/stronger/heavier tongue to replace the bent/broken original. After that, I added a 3/4" 4'x8' deck. It still weighed 400 lbs, far beyond your weight goal.

- enlarging the basic trailer.JPG (96.97 KiB) Viewed 6570 times
* Still, if I had opted to change it over to my previously-planned tent platform (10' x 8', with a separate fold-in half 6'x 8' side extension, with a separate "X-frame" steel underside support - 8' x 8'-) at that point, it might've ended up weighing another 120 lbs for the platform extension, and about 60 lbs for the 1.5' x 2.5" tubing I was using, for the "X-frame" support. The extension would be locked to the base frame side, then folded-out and over the free-standing "X-frame", already assembled, on the ground. Both assembles would be strapped to the trailer for transport to camp, making the total trailer weight somewhere around 580-600 lbs, plus or minus. But, it would've been sturdy, and definitely within my initial goal of 1000 lbs. As it turned out, I commenced with the "hardtop tent" alternative, and today, nearly eight years later, it weighs 2150 lbs travel weight.
* Here's a line drawing of a possible platform for you, though to meet your 300 lb goal, the basic 3.5' x 7' trailer would have to have a folding 9' long (x 3.5' wide folded) piece secured to the top, and either use it centered over the base trailer for support (pinned to it, of course), or use a separate support structure (or just separate legs placed under the extended perimeter). I think you might keep it under 500 lbs, easily, but if you use smaller tires, thinner plywood, no extra steel, unlike me, maybe 300-400 lbs is do-able.

- folding tent base trailer.jpg (256.82 KiB) Viewed 6570 times