With all the talk of mounting things to racks, either permanent or otherwise, I thought I'd post my own approach. Here's my commuter in all its dusty glory. Cost $10 at a garage sale, and over two years I have eventually put on new rims, tires, seat, cables/housing, freewheel, chain, and front/rear derailleur. Total cost has been a bit over $200, but I've learned a lot (which was my point).
As you can see, the height of the toolbox made mounting a rear blinkie difficult. I bolted a hand-rail holder to the box, wrapped and epoxied inner tubing (epoxy is wonderful) around the end of that (to create a platform to mount my light), and mounted my Trek Disco Inferno onto that. Works fine, but I've struggled with vibration loosening the screws. Eventually solved with locktight, but now it points a little downward.