I realised I’d made a rookie error with the original calculations (aka calculating pleats into whole length instead of finished length!). Once that was sorted I got sewing.
This is the finished cage cover. Despite careful measuring for some reason it has one end invert shorter by 2.5cm and a lining pucker under half a pleat. Despite this, it falls nicely allowing plenthy of airflow through the bottom. It’s held to the cage using snap clips over the top bar and the front ties together.
During the daytime, it’ll just be unclipped on each side and the top catcher pan taken out so birdy has plenty of light. The back will remain attached and the sides drop down. If birdy manages to pull the sides close to the cage for a nibble, the end ties are long enough that I can just loosely tie the sides behind the back out of the way.
I’ve included the corrected pattern measurements for any parronts looking to make their own cage cover. This would also work well for a set of curtains, you’d just have to adjust the length to your window.