Here is my method of mounting fans:
1. Four 6/32 x 1.5" nylon machine screws and nuts.
2. Four rubber washers for the screw heads (assuming screw heads are on outside of case). You can use soft silicone grommets that you slice with a razor for washers, but this may be overkill if you follow my procedure.
http://www.earsc.com/grommets.asp.
3. Lay a 1/8" bead of GE 100% Silicone II Caulk on the case where the fan will be attached and let it set up for about 30 minutes. It doesn’t matter much if you are a bit sloppy or uneven in applying the caulk. After 30 minutes, attach the fan with the screws and nuts. Do not over-tighten. This will allow the caulk to conform to the outline of the fan edge, but not let the caulk squeeze completely out.
The materials can be purchased at Home Depot, Lowe’s, or any good hardware store. Regarding the GE 100% Silicone II Caulk, it is all the same regardless of the color, labeling, or packaging.