you can use flattened expanded steel using some wire to hang it from the radiator wings that hold the shell. mild steel is pretty cheap and will rust up nicely. I found 18 ga x.5in opening($13+ship for a 24inx24in) is a really good fit you can fit up to 13 ga but it will be tight tight. measure to make sure but I believe you will need 20x20 for a 28-29. for anyone has a 30 31 its 22x18 on the radiator face. online metals was the only place i found a good price on single pieces.
https://www.onlinemetals.com/en/buy/...steel-expanded