Just a guess Dave but imagine lower locating tabs to allow cover to be lowered into place then fixed at the top. Would removing sidescreen from door help though assuming you haven't tried that?
Morning Richard, yes I did remove the sidescreen, but it makes no difference ref access to the sidescreens alloy locator sections. The bottom of the inner sidescreen panels are very firmly held in place, so I believe there is at least two blind locator tabs ( or something similar ) that are holding it. The tops are held by two alloy sections ( clips) that seem to be also attached to the top of each of the alloy sidescreen locators, each one is held in place by three small hex head fasteners that sit behind the perspex in the windown channel. The rear one is easy to access as its behind the sliding section of the perspex, but the front one is bang in the middle under the front fixed perspex. I have never removed this before and on the CX it has a fair curve on it as it sits in its rubber seal. I am not sure how difficult it is to slide this front section of perspex back to allow access to the three hex screws in the channel, it looks and feels firmly located.
As Paul says a photo of the inner sidescreen panel construction would hugely help as it would show how its all held in place.
Dave