You can only do so much! And bearing in mind that mice can get through holes as small as the diameter of a bic biro (apparently), you'd have your work cut out for you in a shed.
@Badgardener I was thinking along the lines of making a wire mesh cage I can put over the seedling trays as the plants soak up the sun from the shed window.
I didn’t know mice can squeeze that small.
I think it's a bit of an urban myth TBH, but they can definitely get through a half-inch hole.