I like to resurrect this old thread and go back to the original question.
I can get a used RDX450D at a decent price and have a 500D
So you who had these housings please remember back in those days ;-), what can't I do with the RDX450D housing?
I sold my RDX450D to a guy who had a 500D. From what I remember both cameras have exactly the same buttons and button locations and so will fit in the 450D housing. I think the problem comes with the 550D as the camera is ever so slightly bigger.