The good thing about cutting an old tank in half is you can see what's going on inside. This tank is pretty clean, with only light rust, and cleans up well.
Once the tank was cut down the middle I cut two strips of thin sheet and shaped them to fit the contours of the top and tunnel of the tank.
These were tacked on, then the other half tacked in place.
I offered it up to the bike and it fits in place a bit tight, but some gentle reshaping of the tunnel with a hammer should sort it out.

It's wider than I would've liked now, but it was quick and easy and that's the point.