Sealing an air drying box

I'm aiming to build a solar-thermal drying box to reactivate some silica gel.

One part of the project is the air heater, which I'll be managing using a variable speed blower and a bunch of temperature sensors. The part I want to design now is the drying box itself. The basic idea is that hot air is blown into it, which heats the gel, separates the water from the silica gel, and then exits.

What I would like to do is have some "air tight" seals popping into place when the blown air temperature falls below a certain threshold. This way I can leave the device unattended (outdoors, overnight when it hits -5°C) and retrieve the reactivated silica gel when it's finished drying — even in Winter the air can be heated to sufficient temperature in full sunlight.

I'm currently trying to find a suitable "air tight" seal. It doesn't have to withstand much pressure, just a simple metal lid pressing against a soft rubber seal will be enough. My question is whether such a thing is available commercially, that might be activated using eg: 12V from a lead-acid battery, or mechanically in such a way that I can wire up a DIY solenoid. A mechanism that requires work to open or close, but will not require energy to stay open or closed would be ideal. Even a suggestion of a mechanism that I can build myself would be useful.

At present the vents will be in the order of 12cm x 12cm. I haven't started cutting and building yet, so I have the option of designing the box around a suitable vent.