How to make an air filter

https://www.lasko.com/products/20-inch-box-fan-b20201/

https://store.laskoproducts.com/p-1005-20-box-fan-model-b20201.aspx

20 inch blades are a common size for mowers any well stocked hardware store or big box home supply store has them

If it's a plastic mesh filter, you can buy an uncut kit at most home centers and hardware stores. That you can cut to fit your unit.You'll probably have enough left over to make 2 filters.

Go to where your unit is and look for a filter rack take the cover off might be sheet metal that slides up and can be very tough to lift sometimes. You might have to get a tape measure and measure the height and length then go to hardware store or Wally world and it will be 1 inch by ----- by ------. Sometimes you have to get two of them in case the filters are so long that they don't come that long. But for the most part they could be 14 inches by 26 inches. 20 x 20 if you need help get back with me and can help a little more. ken

I would go to an ACE hardware or any other hardware store, bring in the old knob if you had it to find another that would match the shaft which it connects to, another place might be Radio Shack.