I drilled off the stiffeners, marked the cutout, drilled an access hole and removed the centre section with the nibbler and die grinder. I then trimmed away the flange at the front end of the blister and filed it to fit up through the square end of the recess (corresponds to the flat base of the cylinder, but looks better rounded).
When that was done, I measured and drilled for soft rivets, squeezed and applied a smear of JB Weld around the seam where the ally meets the blister. Later, I'll sand this smooth, prime and paint. Next I'll make supporting mounts for the cylinder underneath, then get to work adjusting the clearance of the flap torque tube and it's carbon fibre cover.