Just finished making a homemade cabin air filter using a furnace filter (I used a 3m Filtrete with a 1500 rating). Thin pieces of cardboard with silicone to hold the filter (it fits snugly in the plastic frame) and a bead of silicone for additional holding power. If it's cut carefully it ends up the same as the stock filter (not quite as deep, but it has more pleats) with a cool looking wire mesh cover (need the mesh to support the pleats). After replacing leaky fuel return lines ($10/ft at VW dealer - no way), using weather stripping under the hood, and replacing A/C drain valve (valve flap removed by previous owner) I still have a hint of dank basement through my air vents. If this new filter works it will be less restrictive, and bring in more fresh air without having to use the fan. It's not very durable, but at 1/4 the cost (it's 20x25x1) I can afford to change it frequently. For added durability I could have used pieces of plastic cardboard (bought at a home improvement store). Silicon seems to stick to it well. My biggest concern is sucking the filter into the fan (not good).
