See this page in thread "electronic filter monitoring" (for XV filter) and following pages for a
Botvac example; different designs possible.
http://www.robotreviews.com/chat/viewtopic.php?p=119605#p119605You can secure a piece of filter material such as furnace filter cutouts with a single ply of tissue paper wrapping around the plastic frame scavanged from discarded filters, extending outside the compartment, secured by fit at the edges.
Different grades of furnace filter are available. I used it just to avoid buying filters. I always saved the frames for DIY filters.
The fancy part of the commercial filters is the fan folding to expand the area, not so easy to make. The larger area is important for the suction air flow superior to the original XV filter.
Links on such topics can be found in the Wiki thread top of forum "Mods & Repairs List".