Rob
After taking a Micrometer to mine, I get .440" which is a 1/4" on the mountable recess, then 2.200" which is 2 3/16" mounting insertion and 2.905" which is 2 7/8" for the outside ring. If you have a micrometer, use it, it’s easier to measure inside dimensions. This is the most common size for a round Hygrometer. As you can see from the included pic, it's the same size as a standard analog. I hope this helps, sorry for being so techish with the specs, but every little bit helps.
If your plate is similar to mine, as in removable. You can take it off and use a jig or band saw with some steady hands and cut it larger. You may know someone who has these saws.
Ask yourself too, is it worth the time and trouble. I would do it, but that’s me. Mine has a plastic grommet inside the opening, yours may have one and you could remove it, possibly making the opening larger. Just a thought!
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