I worked on Canberras as a rigger in the 70's so am aware of the construction of the fishbowl canopy,as well as limited height for the pilots head - it was of 'double bubble' design ... ie 2 separate perspex canopies with an air gap between them.
During the 70's the only chap that 'blew' the canopies at EE unfortunately died and there was nobody else with the experience to do the job...
ISTR that the first few canopies which came through as replacements from the new blower were restricted to day only flying as they were not optically perfect.
From Page 41 of B57 Canberra at war by Robert Mikesh talking about the canopy redesign ....
The original double layered glass would flex with changes in temp and pressure,esp during the ground attack phase of the mission.It therefore became impossible to place a gunsight behind this canopy and have acceptable harmonisation and accuracy.The sight had to be behind a flat glass panel.
There you are gents...for the B(I)8 any gunsight had to be behind flat glass and for the PR9 it gave the pilot a higher seating position without a major redesign of the fwd fuselage.I doubt there was even room for a gunsight to be safely mounted behind a fishbowl canopy !
rgds LR