The message
from "Andrew Mawson" contains
these words:

The sheeting is 4mm polycarbonate twin wall that slides into channels,
and the wind makes it bow so much that it comes out of the channel.

I've worked with twinwall of that size and I understand the problem. The
manufacturers recommend a minimum channel depth (or width) of 18mm to
prevent the collapse of the edges and it doesn't sound as though you
have that much. The frame may have been originally designed for glass.

I would first reinforce the long edges of the sheets, ideally with
something rigid but filling with silicone should be enough to prevent
collapse and give the channel something to grip. Then I would also do as
others have suggested and fix additional cross-pieces inside the frame
to which you can fix the twinwall itself. Clear silicone has pretty good
adhesive qualities, especially if you roughen surfaces a little before
applying it, but a mechanical fixing would probably be a good idea as
well.

HTH