time flies...
As mentionned, I spent some time with my dremel, and think I have a plan...
As the B4 shell is not really prpared for the mid-engine configuration, I am wondering whether I might use a cougar shell instead, so I drew up the outline of a cougar shell on a scrap chassis, and fired up the dremel.
It then ended up something like this:
The plan is then to make some 3-5mm wide c-f reinforcements along the sides.
With the shell, it looks like this:
I just need to make a hole for the slipper adjuster and maybe trim it a little.