What about just getting a roof canopy to fit?
We have an Ifor Williams 8x5, sounds similar to yours, low solid sides, and it had a big roof canopy on (for moving house) then we swapped it for mesh sides for other jobs. It isn't used for livestock although I suspect it could be with a few mods.
We also got an IW canopy/cab for our pick up so I know they come in all shapes and sizes.
IW may only do canopies for their own standard size trailers bur a trailer manufacturer could knock up something to fit properly, with vents etc, and it would be much safer.
Yes don't forget loading gates, ramp angle shouldn't be an issue seeing as sheep go in the double decker jobs easy enough and non-slip flooring.
Failing all that any chance you can borrow one?!