Thank you for your question. You will need bottom plenum that will match the dimensions of the furnace which are the following: 21-1/4" x 21-1/4" x 48-1/4". For the top you will need 11-5/8" x 20". We do have plenum options available. For assistance in finding exactly what you need, please give us a call at 1-888-376-5487.