I designed this 4in cover for my ceiling vent fan. Works great. 

• Buy a 4 in hole saw and make sure the outer diameter is bigger than 100mm.
• Six M3x16 screw for the blades.
• Four wood screws for the 4 corners.
• Mesh and duct base are optional. 
• No support is needed Print the blade with 100% infill.
• Depending on the precision of your 3D printer, a tiny drop of lubricant oil at the M3x16 screws 

Good reading on difference between bolt and screw https://wilsongarner.com/the-difference-between-a-bolt-and-a-screw/