The word just doesn't seem to want to get back into my brain.
I can't even put a picture of it, seeing as I don't know what to search for. Its like an extension boot, open-air, not covered. It's attached to the vehicle by a hook or something similar. It has to be open-air though. Something not as big as a trailer house.
Also, I'm getting slight hints of such a... device being seen in a Charlie Brown Comic?