I respect your decision and interest. To me, the SP is a computer, tablet, and note-taking device. It's expensive, mobile, and in my case - very powerful for my work. But, it is in my opinion, too big for my particular taste for use with my workouts (size & weight & cost of repair).
Therefore, may I suggest an alternative?
If your goal is a WiFi + media consumption device (movies, music, e-reader, and browser) while on the treadmill / bikes - why not spend a little to get a BlackBerry PlayBook ($60-$70 on Craigslist) and you can load Kindle on it for e-reading, it has built-in media player for movies & music as well as a browser. It is 7 inches and while it's a dead device (no more investment in the platform) it is perfect for those types of situations. The cost of it is far less than repair of your SP, IMO. Other choices may include a Kindle Fire or if reading only a mono-Kindle for $40.
If you're diehard on the SP -
eBay and
Amazon have a large selection. Keep in mind, many of those cases don't afford the best airflow that the SP will need if running videos. eReading shouldn't warm it up too much.
If it were me - I'd opt for a smaller, less expensive, lighter device.
Good luck in your quest - both for a case and for your physical health!