Here is another jig I recently built and quite honestly - I really like it. I read about it in GAL issue 73. The article was by Mike Doolin who designed the jig (It amazes me what he can innovate).
http://www.luth.org/backissues/al73-76/al73.htm. I've carved enough neckshafts to know that consistency has been a bit of a problem for me. Also they take more time than I want to spend (this may be the bigger motivation). This jig was more challenging to build than any jig I have built. I bought and machined the aluminum parts twice before I got them accurate enough. You can see in the images that there are two frames - an inner and an outer. The innner slides N to S with 3 or 4 inches travel and the outer slides E to W about 18 inches. You can see that it only carves the straight, tapered part of the neck. I have done 3 necks with it and all three came out fine and took about 5 minutes each, after which they only needed a little sanding with 180 grit to clean off the tiny facets the router bit leaves. I made some things a little different from the article. One is the router bit - I could not find a 3/4 inch diameter one, so I used 1/2 inch. I also used standard kitchen drawer slides, but had to scrap them on the outer frame because they were slightly too short. I kept them on the inner frame. Anyway, enough typing, here are the pics. Full credit to Mike Doolin and for his willingness to share.