I use the same as norty1. I used thin foam self adhesive weather stripping as a gasket around the hole in the ceiling. I did this to help seal any leaks around the filter and help force all the air thru the filters and also to help hold the foam filter in place. Works great.
Hope this idea helps.
2015 Pinnacle 36FBTS
2015 F250 SD Platinum 4x4 - 6.7L PSD :
Titan 50 Gal. Fuel Tank
Slotted and Dimpled Brake Rotors
Air Lift LoadLifter 5000 Ultimate Airbags with WirelessAIR Controller