That is not something someone else can tell you easily.
You understand (I hope) the environmental thermal characteristics you are targeting for operation, the design of your enclosure, and the estimated power dissipation of your device(s). Then your thermal engineer (you do have a thermal engineer on the project, correct?) can run all the numbers and provide recommended solutions to keep your device within operational limits.
If you truly have a closed box (ie, no external air convection) you have a an interesting/difficult problem on your hands.