Brian,
Of courser the "easy" way is to contact Diamond Sea Glaze, and see if they can supply the part.
The other way, is to make one yourself. It will be aluminum. You can get soft aluminum at any Home Depot or Lowes. It comes in both bar stock or angle. Don't remember the dimensions, but the good news, is that you have the port one to use as a guide. There will be a "fur" wiper strip. That is available in various catalogues. (also from Diamond Sea Glaze if you wish).
After fabricating the piece, you can use JB weld to put it in place. One member posted that there is a Rustoleum paint which is very close. I have also found that "Hatteras White" is very close to matching the color. Paint after your have put the piece in--there will be some sanding of the frame, and perhaps you may want to notch the frame to get better stability of the new piece. Sand down the J B weld, prime and paint.