I wouldnt bother they end up holding the moisture to the van and rotting them out ,pointless if you have a garage.
the cloth/cotton ones or dust covers are ok but best to use with a dehumidifier,with ventilation if you have a show winner. I would imagine you could get one custom made from dust sheet?? if theirs no one out their that manufactures them wholesale??
Dust is a minor issue compared to damp. A dehumidifier as a great investment for a workshop / garage.