I was able to find a 13x22" sheet at Joann Fabric for $2.50, label pic below.I bought the one called "Plastic Canvas 12x18-Clear" for $5.83. I ordered other things as well so I didn't have to pay shipping. The length doesn't quite reach across the diagonal length of the incubator. It's short about a third of an inch or so on both sides but this isn't an issue because the shape of the incubator holds it in place and the gap isn't big enough for a chick.
Any decent sized chain hobby place should have them, I had to drive 30 miles.
Five bucks at amazon with free shipping.
It's a '5 mesh', largest holes made (I think) for max airflow.
As far as stiffness, the material isn't indicated. Mine's pretty flexible too.Preferably look for sheets that are stiff. Mine is very flexible, but still works.
Gluing it to a base and where the 2 pieces interlock will stiffen it up.
The flexibility at the top curve is actually helpful for it to settle into the corner groove of the lid, once trimmed just right.