Got one of those walk in plastic greenhouses. The floor is just earth and
so I've got a few one and half foot square plywood boards to put down as a
flooring.

Will they last well enough to just lie down straight on the soil or should I
put some plastic sheeting underneath, so they might last longer and not rot
from the damp?

I was thinking of resting them on bricks at each corner, but then thought
that the slugs would just hide underneath. Grateful for any suggestions as
to how you would lay them. Thanks.