LIFT the skirts at both ends of the enclosure..maybe a foot above the ground to allow air circulation inside the enclosure all winter. A little snow on the ground under the boat is not a problem. The enclosure is on moist ground which provides moisture to the interior of the enclosure. This is good for a wooden boat....within limits. Check CONDENSATION from time to time all winter. NO condensation is Good.

NB
BTW:
Earth/Sand/Gravel under the boat is BETTER than a concrete floor under the boat.