I have a feeling this is the approach I'll be taking, I just need to set up a separate area for them before they go outside for the first time.

I'm less concerned about mice and more concerned about for example flies and other winged insects landing on my tomato plants during the few hours that they're outside, laying eggs, making themselves at home, and then spreading to my other indoor plants when I bring the tomatoes that aren't fully hardened off yet back indoors, potentially causing an infestation among my indoor plants. I feel like I am being overly paranoid, but I hope this helps clarify the scenario that I'm asking about.

I see, so you just open up the cold frame for a few hours a day until they're ready for full time?

Thanks for chiming in, although I think we've got our wires crossed. I'm asking about protecting my indoor environment from plants that have been outside for an hour or two during the hardening off phase in the spring, before transplanting them into the ground. I agree about washing stuff before bringing it inside at the end of summer, that is a good idea.

You do this for all crops or just cucumbers?

For all crops or just cucumbers?