OK. The Gasket on the Wynn filter measures 17.5” OD x 12.5” ID, and it needs a flat surface to seal against. To accomplish this you will build yourself a Donut.
Then you can go Here to select the efficiency of your new filter.
How do you build a Donut? Basically it’s just a circle of plywood or MDF, with a 10” hole in the middle. Some good examples of Donuts look like this:
(You can use your Back button to return to the previous screen.)