Thanks so much for this post, I never knew vent fans were a thing. And once I saw this and started to research it, I found there are fans integrated to vent covers with built in thermostats, like this one:
So I am lazy and will try one of these since it has a thermostat and can be set to run only when the AC is on. Thanks for the post, very helpful!
The problem I have is that my vent is a ceiling vent. And it is a strange size, 6"x6". I have not been able to find a vent fan that will fit it. 
Mine arrived today. Will let you know how it works.
Mine is in the ceiling too but fortunately near the wall. It is also in my craft room which isn’t frequented by guests so I’m not too worried about a cord hanging down.
Yeah, my room that needs it is my office. I am not that worried about the cords either. It is just I cannot find any thermostat-fans that will fit the 6" square…
The Oak would be a lot stronger than what I have but the key is one bigger than the opening and two small enough to fit in the hole, making the openings 3/4 inch thick that make it quite strong. I then used the foamy style Gorilla glue and left a powerchair parked on it overnight.
Do you have access to the ducting in the ceiling above?
An inline duct fan might be your answer. Not the “fancy” kind you see here for exhaust ducting, but they make cheap, low power, constant use versions for HVAC applications.
I installed one in the ducting to my office off the main wing of a house, it still runs to this day, probably 15 years later. Keeps air moving and adds flow when AC or heat is on. Night and day difference.
Not sure if I have access to the duct or not. I have not tried to see if can get to it through the attic. But, that sounds like an excellent solution. I will have to look into that!
Doesn’t need to be close to the vent, so anywhere you can tap into the ducting heading in that direction will work. You just need power.
@bill.m.davis I have seen your builds and you have seen my ability, surely you could build something like the OP???
Proud of myself, I didn’t have a standard size vent so I had to improvise. I thought I saw someone else post their adaptor similar to this but I couldn’t find it. My vent was over 13" but the closest booster fan I could find was 12". This is what I came up with, which I will have to admit, I think it is my first project that my husband was actually impressed by 
Adaptor for the booster fan for my vent.
Booster fan installed into the adaptor.
View from the back.
Installed into the ceiling.
@Pearl, that is awesome!
The problem I have is finding one small enough (that my wife approves of). I think the booster in the vent will probably work best for me (suggested by @eflyguy). But, it is far to hot to climb into the attic and check it out for now.
Do it early in the morning.
It looks great! We just got one of two done yesterday, it was a giant hassle because someone (not me) may have damaged the drywall trying to run the cord into the attic to keep it from showing. I have to say, it does seem to help a lot, and we have another one to put up. I don’t think that room will ever be the same temp as the rest of the upstairs, but even if it’s only 5 degrees warmer, that’s actually a lot better than before.
