well i suggest you become a fan of fans otherwise your asking an impossible to answer question without modifying and invalidating your castle warranty.

you could remove the castle heatsink and put a larger one on or alternatively buy a more efficient motor like a neu motor or similar.

you could always pickup a mtroniks controller and using thermal epoxy attach it to your chassis for sinking.

other than that until the mmm your Donald ducked if you don't want to use fans.