I have a Rheem Classic 90 Plus furnace (RCBA-3765 or 4882?) circa 2001. Inside the top of the furnace there are two 12X20 in. plastic filters that came with the furnace. Access to these filters is through the furnace's top front removable panel. My question is about the common corrigated filters that are sold in stores to remove dust particles. My system does not have these filters. Can I put these corrigated dust filters on top of the plastic filters that came with the furnace? Or can they replace the plastic filters? Or do I have to have the furnace inlet ducting modified to provide a place to insert the corrigated dust filters? Thank you.