Summary

This is the remix of the top cover of the really nice Caddyx Vist Case with additional 25mm fan 12V on top. I'm using the Caddx Vista on my RC Car and the Cam overheat really fast.
Had a spare 25mm fan and used it on top of the case for additional cooling.
My fan is 25x25x10 mm and bit bulky. A 25x25x7 mm fan would be nicer and could be pushed more into the case (inside clearance between top of Caddy Vista and case top is around 7 mm). But this needs a bit redesign, a snap-in holder instead of the screws would be nice as well.... ;) I used metal screws which makes this even heavier, but as this is intended for a R/C car, weight doesn't really matter.
The fan cover I used is this one: https://www.thingiverse.com/thing:3887481

EDIT:

Caddx Vista Case Mod Base V2.stl:
Strengthed the side walls between the cutouts a bit; original version breaks easily.

Caddx Vista Case Mod Top V2.stl:
Optimized the Top Cover, the cooling is now better as the fan is closer to the Caddx Vista. The fan does not stick out so much which lowers the center of gravity a bit.
Sealed the fan with some hot glue from below to prevent water getting in.

Also created two covers for the right and left side as I was driving in rain and I think those sides are prone to water splash. Easy to remove to access the USB port or when more cooling is needed.
Covers should be printed with flexible filament.

EDIT 2:

Added a cover for the back side and a sealing ring for the gap between fan and Top Cover.
Caddx_Vista_CaseMod-_Water_sealing.stl
Caddx_Vista_CaseMod-_Back_Cover.stl

I recommend you to mount the fan upside down. In this case, the air flow is optimized. Fresh air will be sucked in from the front, passing the camera and the the caddx vista unit and sucking the hot air out from the case - if the side and rear intake holes are covered.
Mounted like this, I can run the Caddx Vista at 200 mW without any overheating messages just by using a normal 12V fan, running at full speed (3s Lipo).
Different Story at 8.4 V (2s); fan start spinning, but too slow to keep the system cool for a longer time period.
If you need to buy a fan, I would suggest to go for one specially made for RC cars. They have a much higher RPM compared to normal computer fans.