Problem #1:::
You're going to have to support the router squarely on the top of the bass body to keep the pocket walls parallel to the already cut pocket - a big problem when the body doesn't stand out longer than the body itself.
Here's a pix of what I do when I am cutting a pocket - maybe you can understand from this:::
This IS a Jaguar profile and you need to have some parallel support for the router - which is gone now since the pocket is complete on your bass.
You CAN manufacture a support that is level to the height of the bass and parallel to the pocket to hold the router square and true. A complicated, but not impossible job.
You can see here what is needed unless you want to build a full-on jig for this one-off job:::
The end of the body will be at the arrows I drew, but you need the support out beyond the heel pocket - is that easy enough to understand?
This might be a little more help to understand the dynamics of what you need:::
The pen line is where the actual body will end which is not where the pattern/jig needs to go.
Problem #2:::
Since the body is formed and curved and there's not a lot of flat and square surface on it for indexing the router or the pattern, you CAN build an over-the-top support and pattern where the router sits
over the body, not touching it at all.
For that you need to suspend the router from a framework that it can slide on and follow a pattern - in this case you already have an existing heel pocket that the bearing on the pattern-following cutter can trace, but you'll will have to buy a longer router pattern-following bit.
This last idea needs a longer router bit that can reach through the holding device you have to make (it's not available at Wal-Mart of Lowe's) and those longer pattern-following bits, besides being somewhat expensive are also kinda rare and hard to find.
A hammer and a wood chisel begins to sound good right about now.
I made the mistake once in not cutting the neck pocket out at the right time - a big mess and lots of re-engineering and trying to figure out just how to do it ensued at that point.
I got it done but I had to make a one-off support bracket that sorta worked - just barely!