I found some of these at work and they have worked really well , and easy to work with . and build off of .
(http://i927.photobucket.com/albums/ad119/mf5117/tunnel.jpg)
i like the way you think. that tunnel is low enough to remind me of the restricted clearances once common in the east. i cut down tunnel portals so they just clear my tallest cars to achieve the same effect.