Thanks for the link to McMaster-Carr. Looks like they have exactly what I need! The current pattern is diamond openings (as opposed to staggered holes). I suppose that has better air flow that the sheets with round holes in them, although I doubt is matters much.
Dealer did not stock the seal, but it will be in on Thursday. Total cost was $21, including shipping. Not bad.
The diamond 1/2" opening in a 24" x 24" sheet is $9.95 at home depot. It is 0.125 thick which is 3+ mm.
Since your broken frame is plastic have you considered putting a couple pins between the broken sides and doing a Stu bondo job on it?
You could even make a plaster of paris cast split with aluminum foil from the non broken part and do the final bondo shape from the mold.