Easy! Make a template with the biggest and most obvious cavities already on it...
Why?
Because if you screw up the cavity part, the 'outline' part of the template is still useful and THEN you can make separate templates. If you do it all separately, mind you, there's the advantage of being able to use the templates on different styles of guitars (though, if you're creative, you can STILL use the 'tele' body and just the cavity portions).
The way I see it, you get a 'free' shot at doing it right. If you pull it off first try, great-- you have a most-in-one template. But if you don't, well, there's always other plywood/mdf/whatever to give it another go.
I wouldn't put the neck cavity on it, though. I'd use one of the recommended methods similar to those used by the famous "neck pickup jigs", even if you don't build a full-time jig. A makeshift one-off jig is still a better idea, IMO.
Greg