Another idea; there's a trailer lot at route 30, route 128/I95 and I90 (accessible from 30, across from R Glenfield St, and from the on-ramp to 90 from 128 south or River Rd). I've no idea what the rules are for using that lot.

(Google maps puts it at River Rd & Interstate 90 & Interstate 95, Weston, MA 02493 or 42.342281 N, -71.265961W)

It might also be worth contacting a local school