We have some rooms in our house that never seem to cool down because the A/C doesn't seem to move enough air. I've been told to make sure the filter is clean and NOT one of those pleated hypoallergenic ones, (cheap free-flowing is better for a/c), open all the vents and leave the thermostat fan switch in the ON position; notAUTO. Some rooms just need a little boost though, so we found this little fan that fits over the vent. It works great but cost $35.
I started withe a 4" fan and a plastic tub that fit over the register.
Find a fan that you can take apart or it'll be more work if you have to drill and pop-rivit it back together.
I had to drill rivits and remove the stand (watch your fingers...)
I attached the cover to the inside of the tub and carefully cut out the inside, leaving a little extra around the screw holes.
After re-assembling the fan, I drilled a hole for the cord.
The finished product with cool feet.