Measuring thickness with closed box

[deadedith]

Okay so I've got a little side project I'm working on, an older guitar, and I want to sand the top a bit. I know it's too thick.
Does anyone have an idea how to do that measurement other than a magnet type setup?
I have digital calipers and was thinking about gluing some extension on the calipers to reach a bit beyond the soundhole, and building up one end of an extension about 3/4" to clear the bracing.
It could be a stupid idea and I would rather try something that has already been done.
\Got ideas?
Thanks
Dave B

[deadedith]

I found this behind some stuff, I obviously do not use it much, so a little modification might do the trick. One trick only, though, as the reach does get me beyond the bridge but cannot do anything closer. Hmmm
Dave B