so, how'd you make an external mould for a guitar?

Well, here's how I do it.........

First find a pic on the net of a guitar you like the shape of. For me it has to be a Lowden Jumbo.

Then Project the pic onto the wall with a data projector.

Make sure that the scale length on the projection matches the scale length in real life. Draw round it, changing it as you like. (I'm adding a cutaway.)

Cut the first half out of 3/4 ply. spend some time and get it perfect. Use it as a template to mark out three more.

Rough cut the other 3 pieces of ply about 1cm (1/4") inside the line

Clamp it and use a router to follow the original to cut the other three exactly the same.

the notch at the bottom right is to bend the cutaway........

Then assemble the two halves. I use offcuts as spacers between the top & bottom to get a little more width.

Don't do this in your best clothes though...........