Reddit mentions of Gorilla Carts GOR1001-COM Heavy-Duty Steel Utility Cart with Removable Sides, 1000-lbs. Capacity, Gray

Short(ish) TL;DR: Building a box on top of a utility trailer that will bolt-together, plywood sides, seat with locking storage and will be pulled behind a electric front hub recumbent trike. Not for use on the road, low speeds (~5-10mph) on pretty flat terrain with at most some ruts and dust piles. TTITD Already got the hitch made up and working well enough.

Longer form:

SketchUp file for those that want to follow along that way, otherwise image link.

This Gorilla Carts GOR1001-COM utility cart, going to weld up the frame but in sections that will bolt together. Want to be able to flat(ish) pack this in my storage unit when not using it. Main tube will be 3/4" square steel, the tube that's the extra supports will be recessed 1/2" tub, flush to the inner surface of the 3/4" tube providing ~1/4" for plywood to fit in against and help keep it in place.

All the tabs shown are 1/8", and almost all 1"x1". Debating drilling 1/4" holes and using #12 screw bolts, or drilling 5/8" holes and 1/4" hex bolts.

The seat bottom and back and top may be 3/4" plywood, not sure yet. All the rest will be 1/2" on the sides/back except the bottom (sitting on the mesh of the cart) which will be 1/4". Mostly to prevent little things from dropping out the bottom or stuff coming up from underneath.

The small 4" posts sticking inwards are for square u-bolts from the bottom which correspond to the cart cross beams and are 1/2" tube ~1/4" from the bottom to allow for the plywood.

The footrest in front is detachable and narrower than the full width, hopefully to cut down on people using it to step up as much. I'm slightly worried that if they step on the edge it may overbalance the cart. Having it closer to the center may help with that. If not, I'm going to just remove it and people will have to watch their feet with the tow bar :P

The seats bottom and back will be 2" high density foam covered with some basic cloth, stapled to the bottom of the plywood. The seat bottom may be hinged at front or rear to provide access to the front area where I'm intending to have a battery box and such. Using these type latches and some combo locks. Probably have a bit of 1x2" or similar wood behind the hinge area and latch areas to provide better grip for those.

The back will have a strip at the top/bottom, probably reinforced behind by some 1x2" wood or similar to provide better grip for hinges and latches. Might use the same latches as for the seat, but considering using these. It will swing backwards, and I'll may have a bit of steel cable to keep it from going all the way down, I'll see how it is in practice first.

I'll also get some anchors or similar and put them at the bolt points, and possibly at some of the other extra tabs I have in to help attach and hold the plywood in place. Might use screws, might just bolt through the plywood, not sure yet.

So, hopefully I'll get some constructive feedback (probably along with some "you're insane" or "no way that'll work" or what not). And it may not work as I hope. Oh well! I'll still learn a lot about how to do some things, and how not to do some things.

My alternate is just build a rectangular box, hinges in the middle with cushion and fabric all on top split in the middle. Still bolt together as a goal. A lot quicker and simpler and cheaper. But that's boring.

We use a heavy duty traditional wheelbarrow and a 1,200 lb rated four wheeled Gorilla garden cart. Both have their uses. I'm the lady and prefer the Gorilla as it won't tip like the wheelbarrow, but my husband prefers that one.
We also use a towed cart behind our lawn tractor but far less frequently.