Our bedroom is the furthest room away from the furnace and, mostly because of the overly complicated ductwork, it doesn't get as much hot air as it should. I've used those automated register booster fans before and they do help, but they don't make any that fit the size/style of the bedroom register. It finally occurred to me, though, that I could make my own.
I bought six muffin fans (@ $1) that would fit pretty snugly in the vent. I designed and 3D printed a bracket to keep them all in place, and wired them all up to an old power adapter I had lying around. I dropped that into the vent and plugged it into a smart plug.
Usually, those store bought fans have a temperature sensor to tell them when to kick on/off, but I just wrote a rule that's triggered whenever the thermostat changes states. The vent fans kick on when the thermostat switches to heating or cooling, and off when it goes back to idle.
Ten bucks for a smart plug, six for the fans, and maybe 50 cents worth of filament (since it's going inside the vent and won't be seen, that means you can use whatever leftover colors you have lying around) versus $70 for an off-the-shelf fan. Took me less than two weeks to build and a good chunk of that was because my soldering skills are garbage and I had to re-do connections repeatedly.
I don't think it'll suddenly make the bedroom a sauna or anything, but it appears to be at least as effective as the store-bought version and seems to help.
I didn't think to take many pictures, but here's what it looked like before I wired everything up in case anyone's interested...
