Hasta La Motion!

Hello everyone. This is my first post on vfxtalk and I am not realy sure wheter, to put this question in the beginner or professional Talk (I post it here, cause the answer might com from a professional :rolleyes:). I study animation in first semester, and just found out about the great possibilities of matchmoving and 3d. Now my question is(the real question is asked after I explained what I have done so faar), I filmed (some camera movement), standing in an alley, looking on a street (so I got high walls on both sides next to me, and an open end, where the street is passing by). I import the footage into Boujou , 3d-track it and get into my 3d app (maya) , create a car driving on the street. Next step I load it into my compositing app.. And this is where my question appears. How do I mask out the walls on the right and the left, so I see the car driving on the streets only (and not overlapping with the walls)???? One answer I could think of is rebuilding the walls in 3d and not render them out….but is there an other way?????? Thanks a lot and sorry for my english and bad explanation:)