The way I’d approach it would be to stick paper to the (stripped) body so I could map out the position of the neck and pickup. Firstly rough cut the edges so I can run a straight edge along the sides of the body. Then I’d just follow the body lines for the lower horn - that’s the bit that needs to be bang on for me - uniform spacing all around! I taped a pencil to some other objects to make a jig. Repeat until it looks right, then stick to your sheet and get busy with a jigsaw. I then scraped a 45 degree bevel on the plate with a Stanley knife.
I appreciate that might be a bit Heath Robinson for most though!