An Electric guitar or bass is a very easy instrument to make!
I do suggest buying a neck from any manufacturer, or getting a used one on Ebay.
The body can be any shape you dream up, But the neck must be straight, flat, frets spaced correctly, and carved to fit your hand etc.
When you make the body, just make the bridge exactly the same distance from the 12th fret to the nut. Place the pickups in line with the strings. Bolt on the neck and wire it up!