Hi TNM welcome to Dakka
First off it will depend on how you want the looted Rhino to look, do you want an open topped troop transport, like a stand in for the Trukk or do you want it to have a mahosive great gun and turn it into a boom wagon?
This is a link to the one which Forgeworld sell
http://www.forgeworld.co.uk/Warhammer-40000/Orks/ORK-LOOTED-RHINO-COMPLETE-KIT.html
What I have done in the past is to pretty much build the vehicle as intended and then take a pait of clippers, a drill and a craft knife to it to make it look properly battle damaged, then i will have a root through my box of left over bits from Ork kits and add them on somewhat haphazardly to represent the sort of ad-hoc repairs an Ork Mek would make.
One thing to bear in mind is that Ork engineering is brutal and practicle, it is not elegant or particularly neat, so say for instance you have hacked a big hole in the side of the Rhino to represent battle damage, if you cover this over do so with an oddly sized, badly cut sheet of material so that is looks rough and ready.
Another good way to "Orkify" your rhino is to get one of your Boyz, cut his legs off at the waist and have him leaning out of the vehicles top hatch firing his slugga or bellowing orders to the lads.