An acoustic bass would actually be much easier than an archtop, I would think. An acoustic bass, is effectively, a larger bodied acoustic with thicker bracing. An archtop, otoh, is A LOT more work, cause you have to shape both the inside and the outside of the carve, which has to be tuned, the way the neck matches the curve of the top, setting the angle, so the strings have decent action. Theres a LOT to consider there. I'm personally playing with the idea of an acoustic bass for my first project, but they really do have to be amplified to be heard at a decent level compared to any other instruments. If you are just looking for something different, find a guitar that's close to what you want, and use a vector path drawing program (like Illustrator, but there are free ones, I think) to trace everything out. You can then tweak the paths to get exactly what you want. I've started drawing one up using Steve Vais acoustic (from Ibanez), cause it has a single cut away and 24 frets (pretty rare for an acoustic). Best of luck in what ever you choose to do though .