Get a neck (like the Mighty Mite, or whatever), and thin it to your specifications. Some fine sandpaper cupped in your hand and run up and down the neck will shape it to your hand as if you had played it for years and years...
+1..... I asked Warmoth about doing this to one of their necks. I figured that since it's so heavy duty with reinforcement bars, that it could handle being slimmed down. Warmoth told me "just don't sand down so far that the rods start showing through". Looking at the cross section drawing of their bass neck on their website, it looks like you'd have to be exceptionally inebriated weilding a coarse-grit belt sander for that to happen, but I'm sure Warmoth has heard of every kooky misguided thing in the book people have tried doing that voided their warranty. You'd definitely want to put a hard finish on it (Tru Oil is a great feeling satin finish and Warmoth considers multiple coats of it to be a hard finish for their warranty purposes).