4

Just do a search on Ebay for “ZWB2” 20mm (20.5mm actual size, but 20mm fits fine). Just checked yesterday and found one for $0.99 (not posting link because they change all the time). Then again, why not: https://www.ebay.com/itm/ZWB2-Ultraviolet-UV-Band-Pass-Filter-Diameter-20-5mm-Thickness-2mm/263374354991?hash=item3d5255ba2f:g:7kEAAOSwsYpaKjFM