Mount fans on outside of the case?

[FRD]

Hi y'all,

On my case I still have space left for 2 fans on top. However, the case I have is designed to have 2x 120mm fans or a 240mm radiator mounted on the outside. Inside is not possible anymore, since the motherboard reaches the top/ceiling of the case.

Since I still have 2 unused fans laying around, originally planned to put there, I was wondering about this. 

How to mount the fans on the outside? I'm looking for a solution so the fans won't just stick out in the open with the blades uncovered and so it doesn't gather dust when the PC is off. The aesthetics aren't too pretty also. I also noticed that this mount needs the long fan screws to reach, which I'm out of.

Do they sell something like fan covers for the entire fan to keep dust away? How to make the fans sticking out look better and keep dust away? I would like to hear your suggestions. 

[Analog]

Silverstone had this for a while. It is called the FF121: 

 

 

It is a sort of fan grille, cover and dust filter at the same time. Not sure if they are still available though. Haven't seen them available in my region for some time. Might be worth checking on ebay. 

[Owl_unboxing]

i have used fan grills in the past.  Try to search them as "Computer Fan Grills" or "Computer Fan Filters". You will find dozens (amazon, ebay etc).