The one I have a pic of like that is basically a front and rear fascia with a hood on top. There is no space inside to speak of. If someone took a mold of one and made it all fibreglass it would be a bit easier to make, but still tiny.
You could get as much space with a tub on a rack stuck in the receiver.
I think the ones with the roof section attached look dumb.
I don't drive slow, I'm just cautious. I no longer have a supply of axles.