Do you believe everything you read on the internet? I know that I do. But on this subject, there seem to be two prevalent opinions:

• HVAC pros, mechanical engineers, and self-appointed know-it-alls who insist that the whole approach of using booster fans at forced air furnace vents is doomed to failure. It just can't possibly work because of several reasons. The reasons might be entirely correct, or they might be so much hot air (you should pardon the expression). There are similar opinions about inline duct booster fans.
• People who have actually used register boosters and reported that it made a big difference in their homes. I mean, are we supposed to believe them just because they tried it and were satisfied? Humbug!

To be quite honest, I don't know if either of those schools of thought are correct when it comes to my own home. The people who are naysayers might actually know what they are talking about. The people who reported success might just be lucky and got good results for other reasons or despite the theoretical impediments. 

One thing that seems to be almost universally agreed, at least by anyone whose comments make it seem like they know anything about anything, is that it's a loser's game to permanently close off some registers in an attempt to force more air to others. (I'm not expert enough in HVAC to say whether this is correct thinking. I can only say that it's widely believed.) Why, then, do vent registers have adjustable louvres?  Apparently, it's so you can temporarily prevent or allow airflow into that particular room, even if it doesn't dramatically affect flow into others. For example, you might want to only heat some rooms when you expect to be using them, to save heating costs.

It's inconvenient to retrofit inline duct booster fans in my house. The rooms that need help have the ducts among floor joints or walls. The disruption to just try something is too great for me. On the other hand, trying booster fans at register* locations is just a matter of a tolerable amount of money and time.

(*Why are these vent covers called registers? Beats me. The term was unfamiliar to my adult children, but it's what I've always called them. Web searching didn't turn out a plausible origin within the limits of my attention span.)

There are a few commercially available "register boosters" on the market, and my first thought was to try one of those in one of the rooms to see if it worked. These typically have 1 or 2 fans alleged to be "very quiet" and a power cord or wall wart that plugs into a standard household mains outlet. That's not the best aesthetics, but it would be OK in the particular rooms where I need some help.

• Some are simple on-or-off devices. These devices by Tjernlund are representative examples: https://www.tjernlund.com/ductboost.htm. They have an on/off switch, but you could use a timer or other control at the power outlet so that they didn't need manual attention.
• Some sit above the existing register, and some replace the register with the bulk of it sitting inside the vent. These devices from Suncourt are representative examples of both: https://suncourt.com/collections/register-booster-fans
• The register boosters that are not always-on typically have some kind of temperature sensor (and a mode switch) that will be able to detect the warm or cool air coming from the furnace and use that as a signal to turn the boosting fan on or off. They give some control over the threshold points and the resulting fan speed. These models from AC Infinity are examples: https://acinfinity.com/register-booster-fans/.
• There is this model from SmartCocoon: https://mysmartcocoon.com/. It fits inside the vent and allows you to keep your existing register. All of the controls are via a phone app. You can't do anything without your phone except unplug the danged thing. This is the completely wrong balance point in the user experience for me. I want the occupants of those rooms to be able to make adjustments without having to call the IT help desk (aka, me).
• I found one model that operates completely on battery power, Flair: https://flair.co/ You could be forgiven for thinking running those booster fans for months on a few batteries is a modern power miracle. It takes a little bit of paying attention to figure out that it's not a booster at all. It's just an automated way of opening and closing the louvres on the register. It's a pretty expensive and overly complicated way to avoid the manual task of adjusting the louvres yourself. (And, as we saw above, it's not something you can do in one location as a way to boost another location.)

With all these market choices, why think about building my own? I don't know if it's part of the general supply chain problems or something else, but the choices I thought of as possibilities are actually kind of hard to find in stock except at dramatically marked-up prices. Maybe some container ship will arrive soon and some of them will be readily available, in which case I might abandon this DIY effort and try one.