Try charcoal. I help with work on house flipping and cat smell was a bit bad at one place. A couple bags of charcoal was purchased and set in shallow boxes around including at the return air spot. within a few weeks and of course after cleaning, the odor had dissipated and upon completion of all the work before the sale, you would never know the odor was there. I also used it to get rid of the musty smell in the truck after the floor got wet, put a pop case with charcoal in it under the seat for a couple weeks and odor was absorbed. Good Luck & Be Safe ! ! !