Some way to get them encased in plastic would accomplish the minigreenhouse effect.

If they are caged (I am referring to 5' tall wire mesh cages), you could wrap individual cages in plastic, or wrap numerous plants in plastic. Place bricks or cement blocks or jugs of water between plants to warm up during day, and release heat at night, into the mini greenhouse.

If they are staked, or lying down, add some stakes to support plastic covering. Again, use bricks, blocks, and/or jugs to help with heat.

Keep plastic on during day too may help heat up the environment, but make sure the plants can breath.

The bricks, blocks, and/or jugs idea would help with some heat during the night even if not covered.

Place tiki torches throughout tomato plants to keep area warm enough to prevent frost. I don't know, saw this with grapes recently in a movie.

My in-laws let there tomatoes sprawl on the ground and then when a frost is suspected they throw plastic sheets and canvas painters tarps, and those blue nylon tarps over the area at around sunset. They extend their harvest by several weeks, and it's simple and cheap (if you have the tarps).

You could also build a portable hoop house (see link below), and set it up over the tomatoes.

Good Luck,
Ken

Here is a link that might be useful: PVC Hoop House

I really like the hoop house idea. I will try it and let you know how it turns out. Thanks for your help.