Dave Humby said:
Our conservatory doors are held in the open position by regular 'cabin' door hooks.
I use cabin hooks too. Easy to fit and look good too with the brass finish to match the door handles.
This is what I have on my greenhouse door. Unlike a cabin hook, you do not have to bend down or have a free hand to use it.https://stedall.co.uk/product/rubber-door-holder/
How about using a long bungee elastic cord wrapped round handle and then hook it onto something which you fix up outside near door ie hook.