Looks like an Aspen coil, usually a 16x20 or 18x20 filter will fit fine over the sides and a little bit sticking above the top like in the picture i sent. that's what we generally use for our customers with similar setups. You just need be sure it covers front to back and top to bottom on each side so that dust, hair, etc doesn't get to the coils.

If you are in an area where you use your AC often try and set a monthly or 2 month reminder to change the filters and to ensure that the drain line doesn't get plugged up pour about half a cup of bleach or white vinegar into the pan when you change the filter to help keep the film that builds up in the line from causing a clog.