Fiberglass is what mice love...speaking from experience,,,living in the woods.
A really good solution is to use blue or pink Styrofoam sheets at any home center or lumber yard. Cut the sheets to fit and seal with any spray foam in a can. They also sell glue to fasten the sheets . Use what are called roof disks and some long enough nails to hold them in place till the glue dries.
Cover that with some plywood or metal and your done....warmer floor.
Tell us how you make out.