MIMF

Does anyone know where I would find specs for making a baroque cello bow?

I don't recall whether there's a cello bow in this book, and it's in German, but it does have measured drawings for a number of early bows, mostly German. It's probably still in print, but hopefully you can find it at a nearby university library or via interlibrary loan.

https://www.worldcat.org/title/42437055

Many European museums now have photos of their materials online. These typically have a ruler in the picture for scale which isn't much help to a bow maker but it's better than nothing.

What I believe is now called the Musee de la Musique in Paris has an unreasonable amount of bow pictures online, a little hard to navigate but worth it. I think it's Brussels that has some interesting bows as well.

BUT museums aren't the best at identifying their holdings, so to judge whether something is in fact a cello bow, and more so what era it's from is a challenge.

You might start by finding out what models the more conservative makers of baroque bows are using for cello, and then see if you can track down information for that bow. I would look at Airenti and maybe Hans Reinerts' old site can still be found via the internet archive wayback machine. Eitan Hoffer seems pretty meticulous about being historical also.