I have been thinking about doing a Ubass type build for a while, as well. Maybe you could use the fretfind website to get the location of the frets, then use the stewmac fret saw miter box/saw (or something similar) to get the slots perfect? The stewmac stuff isn't cheap, but the reviews it gets are great.
I'd be interested in seeing any progress you make on the build. Post some pics if you can! (please and thank you