If you want to be creative make a Rocket Launcher like the old shoulder fired launcher with a feeder on the rear and a single tube, just enlarge the feeder a bit and make one larger launcher tube. Shouldn't be to difficult either, just finding cool looking rockets for the feeder might be a bit difficult.
Or you could model it after the MK 19 Army grenade launcher, but you would have to have a marine firing it.
OR.... *gasp* if you are feeling really froggy seal the door closed on the back of the Rhino and take a fine saw once the entire model is glued together and cut about from about 1/3 to 1/2 of the model to about half the model down, take plasti-card and seal the open area (you can leave gaps for hoses and such) and create a lifting mechanism to hold the launcher in place (or make hydraulic cylinders) and create box that raises up from the back of the rhino much like the US Army's MLRS unit.