I recently purchased a 13 year old home with a Lennox G16 type furnace. This is an upflow furnace that uses a hammock filter. When I went to replace the filters in it, I discovered that the boob who owned the house before me had discarded the hammock filter frame and media and placed a slab type filter over the air intake window. I have two G16 type furnaces. Can anyone tell me how I can replace these hammock media frames?