Basickly its like a plastic filter, most are used for fish tanks, BUT if you can get the right size and cut it to shape, you can make plenum foam air filters that will last and wont collapse and rot
 
Most of it is too soft more like a sponge... its like the fish tank stuff but its not
 
Ebay, activated carbon foam in sheets or packs for aquariums, but they aren't the same thing, they're more material rather than chemical traps.

ebay foam
 
If you are replacing the plenum filter might be worth buying some replacement screws for the scuttle panel, mine were all rusty
 
My screws are fine, I dip them while I fix it, and dab copper on the screw before I put them back in
 
With my screws I generally have a different lubricant . Supplied free of charge
 
I have bought one, it will do 2 filters (of a Chinese eBayer) comes to £1 with the post and packing, so whats to lose, I will see what it like and report back if its ok or not when I get it, if it works and is ok, then thats 50p each!!!
 

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