None of the available slug repellents work properly, and they tend to be counter-productive if they get wet.
Anyway, it should be possible to make a cold frame slug-proof (snail proof is easy) - at least against slugs coming in over the surface - until the weather warms up and you need ventilation.
It also fails against slugs from below - especially the keeled black ones that can be only 1mm long.
Putting a capillary tray(s) raised above the ground will much reduce problems from below - but slugs and snails coming in through essential ventilation gaps will still be an occasional problem.
Basically you are always going to need quite a lot of some kind of slug pellet to get the population down to manageable levels reducing to just a few to deal with incoming...
The alternative is using a fine mesh cloche instead - it's much better at ventilating itself. The downside is that rain gets in so your emergency backup slug pellets will only last a few days in situ whereas the ones under a proper cover can last for months waiting for their prey...
I suppose a mesh cloche under a raised open car-port type roof might be the best of both worlds. This type of open-sided roof is also very good against blight provided the tomatoes are well away from the edges (and you don't suffer from horizontal rain).
Cheers.