So I got some old Rhinos in a trade, from the Rogue Trader era indeed. The trick with this is that Rhinos in generally are pretty small compared to the actual troops that are meant to get in them. But the Rogue Trader ones are even smaller!
So I made my first venture into plasticard. I raised the hull up and then extended it. It needs a lot of tidying up and all the extra hull with need some more trim on it, including removing the first attempt at trim on the rear of the vehicle.