MorganS:
If $17 is too expensive then I suspect that two lasers will be out of the budget too. That sensor is also not flexible.
Yeah, but it was fun to conceive of.
[10 Laser Modules for $5 on eBay](http://"10 Laser Modules for $5 on eBay")
And who says it isn't flexible 
Maybe the base bar bends slightly, causing a different sort of change in scattered reflection -- a dip in brightness on both sides that could be registered as different from a side-to-side slide.
Just thinking out loud ;D