I don't know if you're handy with a sewing machine or not, but I went to Home Depot and for around 8 bucks, got a 30"x62" Elastrostatic hammock filter from the furnace/heating dept. and cut them into about 8 individual filter wraps. I swiped some elastic banding from my wife's sewing supplies and fashioned them in about a half hour. If you're in a pinch, the waist band from a couple pairs of old guanchies, one on each end will do just fine. Outer filters for many years to come for less than a buck a piece.
Kevin