[IKE]

I have also often wanted to make one of these. You mention needing 25" of travel. What level of precision and accuracy are you expecting, wanting? If you are looking for something that is accurate within 0.5", that can allow for one level of construction. If you need something within 0.05" or tighter, your best bet may be to stick with a preofessionally made string pot.

Another key techincal point is response time. for instance, a tape measure takes a couple of second to wind up, where as a key/badge retractor may be significantly faster. The pro grade string pots I have worked with in general aren't very reliable past 20 Hz, or 0.05 seconds as you start to excite string modes.

Beside the key or badge retractor, you may want to look into a Yo-Yo ball.