I wish I could point to a bimetallic contraption that would open the lid, but after a little searching I haven't been able to find anything -- perhaps because that solution isn't strong enough to lift a lid? Maybe someone else knows of something in this category that would do the job?
I'll suggest taking a different approach, unfortunately with added complexity. Instead of lifting the lid:
- run electricity to the cold frame
- add louvered vents to two walls
- in one of the vents, add an exhaust fan that is connected to a thermostat
- make sure the other vent is oriented so that the louvers open when the fan is blowing out
Fortunately a hot cold frame is associated with sunlight, so you may be able to use a solar panel instead of connecting the cold frame to the grid.
If you get a lot of snowfall in winter, you'll need to make sure that the louvers do not get obstructed by snow. You'll also need to check regularly (daily?) that the louvers do not get frozen shut.
Also, there's a danger here that if the cold frame is humid during the day and temperatures drop rapidly some evening, the louvers could get stuck open and the interior could drop below freezing. This probably wouldn't be a problem in the dead of winter when all you've got growing are greens (which will generally recover from such a freeze). But it would be a disaster if you get a cold night in mid-spring when you're hardening off peppers or other cold-sensitive plant.