I think the best thing to do for a hallway of this type is to simply let the guests create their own monsters. No, I'm not saying give the guests some play-doh; I mean let their minds sculpt a monster. I would do either of the following:
1) Have speakers set up along either side of the hallway to play crawling/scratching sounds.
2) Have actors behind each side of the hall and have them scratching the plastic. I would assume your garage has a cement/concrete floor, so make some sliding gloves for your actors to use.
The key to making this the least little bit effective is to make sure that it is PITCH BLACK. Since you're using plastic, though, I'm not sure how realistic it would be considering safety...