Eric.
Have a look at GoodHand UK adjustable quick-release toggle clamps, there should be a size and design variant to suit your purpose.
https://www.goodhanduk.co.uk/Catalogue/Toggle-Clamps
I have used their stainless steel adjustable toggle clamps to lock down the storm hatches on a yacht, which are a very similar design to a rolling roof.
For an observatory roof the nickel plated steel versions would suffice though they will take on a surface coating of rust eventually, for long term corrosion resistance the more expensive stainless steel versions are best.
If you search on Amazon or eBay you will find several UK based suppliers of similar toggle clamps.
William.