Haven't finished checking out what the rest of the thread states, but since the first page seems to be copies of the same posts several times, I'll answer now.
Yes, weasels and other "armed" predators can reach thru 1x1" wire - they can also tear chicken wire easily. Some can reach through 1/2"x1/2" (which by the way, totally sucks).
Yes, not only can those gaps let predators in, but your chicks at that age can get thru quite easily. Older chicks can get thru the gaps on them, too. Ask me how I know, LOL!!
Easy to build, temporary, and affordable run. HMMMMM... hoop cattle panels ($22/ea @ TSC). You can attach them to a tire frame, a wood frame, a PVC frame OR you can drive t-posts into the ground to hold the panels where you want them. You could even put 1 side against a tree or 2.
Then you can cover lower portion w/ small enough wire to prevent predators (recommend HW cloth) from getting in and cover the top with at least 2x4 wire to keep your mature chickens in (chicks aren't going to fly 8' high, usually). Or you can roof it with a variety of different roofing products - as simple as a tarp or green house panels or roofing panels. You can add as many panels as you want to make it longer.
Again, a variety of options for the end not attached to your coop. Use cut panel for gate, build a framed out door (if I can do that with a simple circular saw, a handful of screws, 2 hinges, staples from a hand held staple gun and chicken wire, anyone can!!), use PVC...
We use our hooped cattle panels as the actual coops and the chickens free range during the day. I also now have cattle panels hooped over some square pens that were built by the previous owner of this property (have 2 done, need to do the other 2) - so now I can stand up in those pens. We are building fenced areas off of those coops - hoping don't need to cover them (would be pricey - even using hooped panels) but may have to as I have several breeds that I am now breeding that are very flighty even w/o considering predators...
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The feed bags on the bottom of the pen served two purposes - kept "chicks" in the 2x4 wire that was up on the pen when I had to put them there before an out of state run while having someone else take care of lots of young chicks and now they help to provide wind/rain protection. Both of these pics were taken AFTER Hurricane Florence ripped through. Now we have the aftermath of Hurricane Michael (wind and rain, but didn't hang out this time) - both of these survived WELL. I was impressed. Yes, I know. My pens are "kinda hilly billy country" in looks. You can certainly make them look nicer and there are many on this forum who have.
We have had predators and I will be replacing the 2x4 wire (old and rusty, actually) with HW cloth since eventual plan is to have much younger chicks in these pens... The chicks that could get out are 55 Flowery Hens that were 12 weeks old (pullets & cockerels) - body build/size similar to Leghorns.
If you REALLY want to use those pre-built panels - I recommend chain link dog kennels. Wire is actually stronger, gaps are smaller and they are about $300-500 less in cost for the same size pen(s)... You would still need to put some type of wire to prevent predators from getting in and keeping your chicks in at that size..
Snow load - you can still use hooped panels. Search "Blooie". Her and her hubby have a whole series w/ pics of how they built a run in WYO and how it handled snow load.
Good luck with your CornishX as pets. It's good to be working with a vet on that since they aren't usually long lived birds...
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