The back has been a pain to flatten, so i'm going to try a new approach. First I had to see whether I needed to add some wood to the centre back seam.
First, I tacked the repaired top onto the ribs
This allowed me to position the bass on the back to see how my overhangs are looking.
With the top tacked on I can move the corners and ribs around to see if everything will fit. It's been apart so long, not everything lines up nicely. But with a bit of persuasion, I am convinced everything will eventually go back as needed.
I decided to add a thin centre strip. Pictured below is my setup for gluing to one side only. The long hardwood beam holds the back flat to the perspex backing.
I use wedges to clamp the hardwood strip to the side.
The strip will be trimmed down to the width of the back plate, then I'll have another challenge to glue the other side.