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There are tons of mobile apps that simulate altimeter. After losing my altitude awareness on tandem, I practiced with these apps. You set your exit altitude, fall rate, and let it rip.
Android (one I used): https://play.google.com/store/apps/details?id=ravenwerks.divebuddy&hl=en
iOS (no experience with this one): https://itunes.apple.com/us/app/skydive-altimeter/id463197450?mt=8
Edit: Obviously not for a tunnel. But it worked for me at home, on a yoga mat. Practice arch, leg position, and alti check. In a tunnel, I'd say, practice flying. Doing alti checks is kind of a waste of tunnel time.