Using 8mm plywood, then laminate it to help stiffen it up? I would think by the time you do all that it will be above the weight of boxed steel with 1/2 the load bearing ability.
Little bits of plywood and a layer or two of glasscloth & epoxy over it? It's nearly weightless, in my opinion....
I'm not worried about load-bearing ability - the thing will be plenty strong - it is rigidity that's my main concern. But relatively easy to make the box stiffer, if it turns out to be too flexible.
Besides, weight shouldn't be something to worry about too much with a trailer. I mean, why save 0.5-1kg in weight there when, in use, the thing will be loaded with many kilograms of stuff.... (I know, a kilogram is still kilogram.... and every little bit matters.... and it's always the last straw that broke the camel's back....); doesn't make sense to me to go through great lengths to reduce weight of a bike trailer. Doesn't mean it has to be built to withstand an earthquake either, of course. You've still got to haul that thing around by your own power, so some prudence in limiting weight is in order.... As always, lots of compromises to be made. That's the nice thing about building it yourself from scratch, for your own use (as opposed to building it for an 'average' customer, with different people having very different demands and requirements).
Oh, BTW... the 'bent that will be the tractor.... it's not exactly a light beast either.... last time I weighed it, it was at 24.5 kg (54 lbs); that's including the things that I always carry with me, some tools, etc.; I occasionally get some fun out of mentioning that weight to roadies, watch them gasp for air....
... and then accelerate and let them eat dust.
Again, this is Flatland.... weight isn't the problem here it would be in e.g. San Francisco. It's the blasted wind that's my enemy. And even in windless conditions, riding with an, err, 'aerodynamically sub-optimal' trailer behind a bike isn't something you do for fun or when you want to go fast.... It does make for some good exercise though....
Anyway, I think my plan of using plywood & GFRP over it will be the strongest and most rigid solution on a by-weight basis, compared to boxed steel. The thing that has me more worried now are the stainless steel flanges that will be mounted to the box and will hold the wheel axles.... as these will actually be relatively heavy.
REdiculous: yeah, riding a trailer isn't something you do for fun or if you can somehow avoid it. Necessary evil, I suppose.
BTW, a worry with PV panels on a trailer to me would also be theft and vandalism. On occasion, people here seem to get joy, pleasure and satisfaction out of destroying other people's property. Wouldn't be the first time people would smash other people's PV panels just for fun.
People.... will never understand them, I think.