I've got Grangers G-wax natural beeswax. https://www.amazon.com/Grangers-Traditional-Beeswax-Protection-England/dp/B00T9HLXWE?ref_=bl_dp_s_web_3460568011

I use about 1/3 of a can, or more, per application about once a year. I should do it every 6 months with maybe a lighter amount but as I said, I forget.

Instructional video by a hat reviewer: https://www.youtube.com/watch?v=QPSlgiO3pH4 He seems to use Nikwax: https://www.nikwax.com/en-gb/products/productdetail.php?productid=499&activity=

New version: https://www.youtube.com/watch?v=ScRgMBNUlAA Apparently, barmah brand conditioner exists now. Halfway through the video, he mentions shrinking in these types of hats. Never used this before.