Your best bet would be to get some 3/8 or 1/2 inch (or metric equivalent) MDF and make one the size of the body. The you can make measurements to cut the neck pocket and pickup routes. Rough cut those in and then use the router to finish with. Use double-sided tape to attach to the body and have a go at it. If you have some 2x4s you can glue them up and practice on that before doing the actual body.
I can't post links/images yet, so I'll just try to explain this as succinctly as possible, and probably fail miserably. Wish me luck. I'll go into a good bit of detail, you very possibly know much of this already. Sorry if I'm repeating old news lol.
I'll assuming you're referencing a strat or tele neck and pocket, if it's something else, like an ibanez with either a square heel or the newer AANJ, or maybe a set neck like an LP, let me know.
The heel is basically a rectangle, on a strat the edge that faces the body is curved out a bit, and on a tele it's flat. On the other end, the rectangle basically ends and it starts tapering in to the curve of the neck. Usually, the neck pocket ends ~1cm or so from where this taper starts. align this imaginary edge of the neck over some MDF, and trace it out. Make some marks where the last fret is, you'll need those to measure out how deep to set the pocket. Don't just trust the end of the body on that one, it may be too far out if the bridge is already routed. Go ahead and cut out the neck pocket on the template, and a square around it. Enough room to clamp it to the body, and to see the marks for the fret.
Now take note of the distance between the front of the nut where it butts up against the fretboard, and the center of the last fret. Similarly, measure the nut to the 12th fret, and double that. That distance away from the nut is where you want the intonation point to be when your saddles are moved all the way forward (or just a hair in front of your high e, if they dont move). Take the difference between the long length (aka your scale length) and the last fret to the nut, and thats how far you need the marks we made to be from that intonation point on your bridge. Using that number, you can clamp that template onto the body. If the body is already cut out, odds are this lines up nicely with the spot for the neck pocket already, but I like to measure twice (or maybe 7 times or so) and cut once.
There is a wealth of information on this on the huge telecaster builder's forum over on tdpri-dottie-com on the tele home depot forum I have been on that forum for a few years and I've built several guitars. The best method imho has been developed by a fantastic contributor there - J Wells from Albuquerque. Basically you clamp two fairly hard guide boards either side of the neck at the heel end, and clamp another one to the heel. Attach those to a flat sheet of 1/2" or 3/4" MDF or Corian with several screws while the neck is still in the clamps. Once the guides are absolutely securely attached, remove the neck and rout out the space between the guide boards with a pattern bit - ie top bearing-guided. The corner radius on a Fender neck is 1/4" or 6mm, so use the smallest pattern bit you can find, it isn't easy to get them that small. Now check the neck for fit on the flat board, it should be a perfect fit. Now rout the body using that board as a template, calculate the depth you need and make the last cut very shallow to get a good finish. Your pattern bit must be in perfect condition - if the bearing or the cutting edges are even slightly worn, it will not give you the optimum gas-tight fit. You can make tiny adjustments to the fit if necessary by sticking tape to the sides of the template where the bearing runs.
Consider drilling the neck attachment screw holes before you rout the pocket so that the exit wounds get cleaned up in the process.
Another alternative is to make the best approximation template you can, wax the neck, skip it in the loose template and fill the gaps with epoxy putty - the waxing will release the neck.
Read tdpri for a full version with pictures.
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