what I did on a testbody (with 2 different horns): make 2 points, in my case two small pyramid shaped blocks just along the sides of the pocket, as close to the body-egde as i could. This made 2 straight 'swivel' points (sp?), on which i laid down the template and I raised the back of the template with a thin piece of wood to make the angle needed. All I did then was secure the template with a bunch of clamps, thats about it.
I hope you get what I mean, my explanatory skills arent all that, and I made scrap f the body I did it on :/
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About the gap, the neck im making has an angled fretboard/heel at the back, and because of that I'll need to leave some space between the neck and the pickup to get the cavities perpendicular to the centerline. So a gap will show.