I built my own so I could haul 16' stuff in the truck bed.
I used all 2"x1/4" wall tube and 3" channel for the top.
U might make a light duty one w/ 1" pipe and run a 6' piece of all thread thru it and put a top plate out of wood if your trim is fairly light.
Let the all thread go thru the top plate, pipe and your standard hitch tube then use nuts to tighten it up.
Could use 1 in front and 1 in back.
Cabelas has them too,
http://www.cabelas.com/