I'm cheap, a bit of a hoarder, and have a sewing machine that I can manage to make two fabrics "stick" together. It's not real pretty, but for about 10-$20 at the local thrift store (if you don't save all your old bedding), you can buy enough sheets/light blankets to sew up covers for the vehicles that are allergic to dust and bird droppings.
Sheets work OK, but the soft, somewhat fuzzy, blankets that are meant to go between the sheets and comforter are my favorite as they don't allow the dust to filter through as much. For my truck, I make two covers, one for the bed and the other for the cab. This way it will fit in the washer. The 67 Mustang is a one piece, but does push the washing machine capacity. The big old Pontiac will get a two piece, when the time comes, that has an overlap at mid car. At the overlap, I have taken several pieces of scrap cloth, wadded them up, individually, to the size of a golf ball, wrapped it with the first blanket, then the second, and tied a piece of string around it to keep the overlap intact. When it's time to wash it, I cut the strings, wash, dry , and re-apply the connections. Without a sewing machine, the whole thing can be connected with this idea. I have also sewn socks at the corners and in the middle of the long edges to put some weight in them so that the breeze through the shop does not blow the covers off. Blue jean rear pockets work good to and look a little better, although looks are not the covers strong suit! lol This depends on what the thrift store has available or what the home makers tastes are in bedding. I have some flowers and even some "Car's" cartoons.
A layer of plastic can go over the whole thing with a few of the scrap cloth golf ball attachments if your cars are exposed to gritty dust and liquid splatter.
I just can pay the money for car covers!