a simple small stand for the round 41mm hygrometers from amazon, has a 4 mm hole for mounting it where you need it. 
there is lots of clearance to make sure there's not too much resistance so the fit's loose in case it dies and you need to replace it. even mounted face down, not sure why you would want to mount it that way, it holds fine. 
slightly refined but not too fancy. 
steep chamfer for easy print and use. 

41mm hygrometer stand

41mm hydrometer stand v1.stl

20 and 26 mm stand-offs so you can mount stuff to you EK Vader fans like a regular fan. 

Just pop out the plastic spacer and push in the new stand off as shown in the picture. there's a 2 mm ridge that will hold it in place. 

EK Vader fan stad-offs

EK-Vardar 26mm.stl

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41mm hygrometer stand

EK Vader fan stad-offs