I know we don't have the problems here with the same predators you have there. But both my sister and I are worried about hawks and also foxes that can be so determined.
We have also been concerned about cost, and I only have 4 birds, so it is easier for me, but I got around it my making my run longer and thinner, my sister has done the same but longer again for her girls. I will try to post a pic later, but basically it is a long thin run with sheets of Ply for a roof.
This meant I could put a top on it from Plywood, and also I can cover it over with a tarp in winter to help keep it protected from the worst of the weather but my girls still get to come outside undercover from the worst of the weather and safe from predators.
I have a string attached to their pop door and feed it up through the run so I only have to go out and release the string and the door closes for them to be inside safe at night, and in the mornings I just pull up the string again and they can come out into their run. It means that even if I have to go out during the day they are safe, and if the weather is mucky I can just close the door without going into the run myself.
I have also put the run along the side of a hedge, so it has extra protection from the prevailing weather. It was trial and error but has worked for us this year. I know you have a lot more birds but maybe something similar would be easy enough to make out of your large pen so you can roof it. I have also followed advice and buried the wire along the sides to stop any foxes burrowing under to get into the run.
I am sure it is such a worry when you have lost so many to predators.
bets wishes,
Jena.