Seems the guys using the filter Piers recommends/sells, 24-50512, with 40/3B or equivalent sized twins are doing fine with it. Not sucking out oil or sucking in the filter. So technically, any one of the filters on the catalog page that has the same or higher square inches of surface area should also be adequate. This should allow you to pick something that will better fit your space. I'm thinking about the oval shaped 7X10" OD base X 7" OD top X 8" tall. That should fit my space better and has a little more surface area than the 24-50512, so may be able to go with the more restrictive PG7 media and be OK.