go on outerwears website, they have a search where you can put the filter size in.  this one should fit, but you might want to measure the filter yourself.  5 1/2 X8 was on unis website for filter size. 

i came up with this, but might be tight on the filter.  just put in a larger size, they have larger covers.  so i'd give them the measurements of the filter and they can get you a cover. 

20-1872
 WATER REPELLENT PRE-FILTER W/TOP
Part Number: 20-1872
Shape: ROUND
Base Diameter: 5 1/2
Top Diameter: 5 1/2
Height: 9
Material: Water Repellent
Price: $25.95