I'm not sure you can stop them if the underbelly is just coroplast, unless it taste bad to mice.
They can Rats can chew through thin aluminum, and a lot of things.
They don't like the foam we plug holes and gap with, at all.
An opening not much larger than pencil and they can get in or chew it out big enough so they can. Treated plywood maybe.
Maybe seal thee bottom up with the coroplast and then spray it with a epoxy coating with fungicide, sold at paint stores mixed in it.
They use to sell insecticide to ix in paint also and may still but I haven't seen in a while. White pain on ceiling of car port at last home never had bugs or webs on it. Might be possible to mix a rodenticide in it too or something they can't stand.