OK, gang. Here is what I do. I just use my Dremmel to make the hole. I used black tape to seal this one and it worked OK, but I used double stick foam tape on the second one and it looks much nicer. I am testing it now to make sure the foam tape provides the proper seal. With the black tape prototype, when I removed the lid, I felt the 'suction' from the vacuum formed by the seal. So, I am pretty confident that the seal on that model was good. I'll let you know how the double stick foam tape scores.
As for making money at it for a manufacturer, what I would buy is a lot of, say 20 of these hygrometers and a metal punch to punch holes in the lids. Also, a sheet of double stick foam with pre-cut gaskets to seal the hygrometer to the jar lid.
The hygrometers are less than 50 cents. The punch would be well under $5. The foam gaskets, pennies per sheet. This kit might sell for $39 or even $49. The consumer buys the jars themselves (which is the heaviest, bulkiest part to ship with the least profit) and additional hygrometers and foam gaskets could be purchased without the punch tool.
A word on these hygrometers.... some of them are way off. Some as much as 7% or 8%. If you think your weed is 60% and it's really 68% you could be in trouble. However, I have found them to be very consistent. I have over 50 of these as well as some of the higher end Caliber IV $20 models.
What I do is I fill a 1 gallon plastic bad with 69 Boveda packs (that I bought by accident). Then, I place all the hygrometers on the Boveda packs and leave them for a few days for the humidity to balance.
Then, I mark each one as to how much to add or subtract for an accurate reading.
Now, I take these and insert them into the hole in the jar (the have little clips that are designed for this application and snap right into the edges of the jar lid).
Here is one prototype....