Yep. It was a continuation of the central, rear section frame. The internal support was tubular, or 'half round', rivetted to the exterior 'flat' frame, the same width and style as the rear central frame, with a 'Perspex' panel each side. The locking/release latch was central, at the front, and there was normally a 'knock out' panel at the front of the starboard 'Perspex' panel.
The internal tube/'half round' section was approximately half the width of the external, 'flat' frame, so you'd get away with painting a piece of clear tape Cockpit Grey Green, followed by the exterior camouflage colour, placed central on the panel. The 'knock out' panel, if you wanted it, could be scribed into the clear plastic.
NOTE: If referring to the SAM Guide on the 'Mossie', be aware there are quite a number of errors, particularly with photo captions !