Is this panel available aftermarket?

Can't find them?

Take a sheet of thin cardboard and make up template of the size with the folds.

Make them up out of new stock, can press in the ribs with some 1/2" round stock and some strategically places wood blocking in your shop press.

Wow, the price of these replacement parts has doubled In the last 5 years, then there is a big shipping bump to go along with it too.

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Can be fun making the parts if you enjoy that kind of stuff.