I wanted the new box to be long enough for the snyth, and strong enough to hold the amplifier, which is a fair bit heavier that the one I used last year. So I used plywood for the bottom, but good old coroplast for the sides and top. The pieces were "sewn" together with zipties, a technique I also used on the tailbox for my old recumbent bike.
Here's the trailer, with box removed (in foreground) and plywood bottom in for test-fitting.
Note that I had to notch the wood to fit the two vertical posts.
Test-fitting of the snyth:
Nice red coroplast pieces (again from a scavenging job)...
And bolted to plywood floor.
And duct tape to cover and strengthen the joints. Instead of heating and bending the 'plast as I did with the tailbox I simply slit it halfway through and then folded it. Still plenty strong.
The inner upright posts are anchored to the outer horizontal ones through the 'plast.
The front frame tube drops, and the plywood was perhaps not going to hold all the weight at the front without bending and cracking, so a support was needed.
Since the wheel axle support bolts poke up through the frame rails I leveled things off a bit with some blue foam.
Loaded for the winter Community Day gig for the dance class. I added some of the same yellow reflecto stuff I used on the sides of the trike's cargo box.
The box works well, and it's nice to finally have a proper cover instead of always using a tarp. The only drawback to using coroplast for the box is that it won't take a lot of abuse in terms of having awkward / ill-fitting items stuffed inside, but of course doing that sort of thing (like transporting the ski-bike) is what cracked the Rubbermaid bin...
The single-sided wheel mounting makes for easy tube and tire change (a plus in the winter), and a few days later I swapped out the crappy no-name tires for a pair of Schwalbe Big Apples that had done five years duty on the cargo trike (now a little too worn for that heavy use, but still perfectly good for a trailer). The great thing about these tires is that you can run them at low psi (for passive suspension) but the sidewalls do not deform and they still roll well.
2009 update: a winter of towing the heavier amplifier has made me realize that it's just a bit too much for some of the hills I have to climb for the dance group gig. So I went back to the lighter, lesser-powered amp and it's fine. Since it's also smaller this allowed me to lop off about four inches of height from the box, which has the added benefit of making it much easier to get the trailer through my basement door. Yay for various small victories...