Airfix Devastator - Canopy short?

The canopy has been worrying me, I thought it could be a problem: three sections and heavy framing, good prospect for a vac replacement. I cemented all three sections together and then laid it on the fuselage..

rightio, it's a bit short then....
My plan is to add to the rear section with plasticard, sand off the framing and then vac a replacement.