Have you considered building one? I'm considering it. I have bamboo growing on my property, so this design is attractive to me, and it might be light enough.

I think it might be nice for me to be able to carry both of my greyhounds. They're very tall. And they don't like to lie down close together, so the bed would have to be enormous. Maybe on the trailer, they'll be willing to be squooshed together. Still, I think it would have to be at least 5' by 7' for me.

Anyway, with a flatbed design, you can lash a box to it, and it will be like a trunk.