This is one option I'm considering but can't decide if it looks 'cheap'.
The two edges of the body were planed and a hole drilled for where I want the pickup cable to run - sometimes it is easier to do that at this point rather than trying to use a long drill later on in the build (just have to record where the hole is so cavities can be routed in the right place later!!).
I then joined the two halves using three clamps to hold the body together and two g-clamps to keep it aligned. Even at this point, I'm quite pleased with how this ash looks. The piece I bought from the tree surgeon is plenty big enough to make another body from and, if anything, the piece I have left is a bit nicer than this.
This piece has some kind of 'feature' in it but that will be well out of the way when the body shape is cut.