Routing Tele Bass cavity?

There's a couple of options. You could make a pattern in MDF or something similar using a laminate trimmer router bit then use the pattern and a pattern follower bit to route deeper. However, I have a pattern follower bit that the cutter part is only about 1/2" long which should be short enough that the bearing can just ride on the side if the pickup route. I think it came from woodcraft but there are other options. You do get what you pay for in router bits though.

I'm assuming you have some experience with routers. If not, get some scrap wood and practice.

 

Templates are completely unnecessary. The OP just wants to make an existing route deeper, so the existing route will serve as the template for a pattern follower bit. All that is needed is a router and a pattern follower bit. (And preferably also a drill, unless the router has a plunge attachment.)

With that being said, that does not necessarily make the job easy. The OP will need access to the tools, and will need to be comfortable with using them. It is often safest to pay a professional.

 

I suppose another option is to clear as much wood as possible using a drill and Forstner bit, and then finish the job with a chisel. This method is nearly foolproof, provided that you are careful with your chisel. (You don't want any accidental damage to the top of the bass.)

 

Plunge router with a short pattern bit. Masking tape on the top of the body to prevent scuffs.

 

I agree, the pattern isn’t necessary. That’s why I mentioned the short pattern follower bit.

I was just trying to give some options, plus, I’d probably want to make a pattern from the existing route to have for later but I’m odd that way.

Also, +1 on ctmullins suggestion of taping the body to protect it.