I found an article from a guy (Justin Lam) that built his own.
This was his conclusion:
So after all this, the takeaway might actually be that timing the starter isn’t all that important, since it stays active at its peak for at least a few hours. And if you maintain a somewhat regular feeding schedule and have a relatively stable environment, then you can probably get a good feel for how long it takes your starter to grow (or you can set up a timelapse with your smartphone camera to see how it’s doing).
This was still a fun way to nerd out with baking, since engineers like metrics, and there’s a lot to measure with sourdough bread. Although this gadget might seem superfluous to some, I enjoyed the precision and confidence it gave me in taking out the guesswork of how my starter is doing. There’s two types of bakers in the world: those who go by feel, and those have a 0.01 g resolution kitchen scale. Guess which one I am 😏