This projects shows how to make a 3 string slide guitar from a  single birch board 1x3x30 inches, and bits of scrap. The good thing about this guitar is that there are no frets to worry about, no truss rod, in fact it is really just a plank with a bridge, pickup and some tuners. You can make markers where the frets would go, to help you get your bearings when you are playing it. To save cash you could use a glass bottle for the slide (like in the picture).

Here is the link: 3 string slide guitar