Well I give you credit for creativity, but to steal my friends stolen line, "a horse can pull more than it can carry". Maybe I'm not thinking of the sacks of gravel you're thinking of, but it doesn't strike me that you could carry a very heavy load with this and keep it stable. Mainly due to the height and location of the load. If you lose your footing you're going to tip over and with an awkward weight, no matter how rounded the corners of the box, you're going to dump your load and cause damage to an object. If getting through a tight spot is the issue, why not use a single wheel trailer or a narrow two wheeled trailer and haul your gravel that way?

As far as velomobiles in Florida, yeah, I can see them being too hot. Now if you could find a way to incorporate vents to allow better airflow without compromising the aerodynamics, then I think you would have a good product/project to tackle.

And yes, your iMac fairing does match your blue bike from above... though I have to say your blue bike looks better than the yellow bike by far. Now, while I've never worked with fiber glass so I don't know what difficulties you are facing with it, but why not attempt to make it more smooth? I think with the skills you've picked up thus far you could easily make a shell for your bikes that would really look sweet! Add some sparkled/flecked paint to it and you would really have a one of a kind bike.

V