Route Control cavity on contoured instrument without pin router

I've been meaning to ask the same question regarding neck and pickup pockets, for a few carved or arched top bodies. Subbed for learnin'.

 

I'd set up a template shimmed flat over the bass and route through that with a template bit with a top bearing. Is the back of the bass flat? If so, you know it will lay flat on a workbench. Just make a MDF template thats tight to the highest point of the top, and shim it level all around.

 

Double sided tape, shims and a handheld router.

 

This is where collar routing shines. Attach a steel collar to the router base. It's a steel sleeve that surrounds the router bit, extending down about 1/2". You can buy collar kits to fit most popular routers. Or you can build your own router base with a collar on it, a sleeve of aluminum tubing epoxied in.

With collar routing, the opening in the template has to be cut larger in size all around, an "offset" to match the difference in size between the collar diameter and the bit diameter. For example, if you are using a 1/2" bit and a 1" collar, then you need an offset of 1/4". The opening in your template needs to be 1/4" larger all around.

Fasten the template to the body with clamps, screws, or tape. Make it level, resting on the highest part of the body contour, and using blocks or shims under it where the contour slopes down. The template has to be secure.

Now do the routing. The collar on the base rides around in the template; the bit never touches the template. Make the first pass with the bit only 1/8" deep into the highest area. Keep making passes, going 1/8" deeper each pass, until you reach the final depth that you want.