I have found a site that has "open cell foam" that comes in a fixed length... obviously you cut to fit.

Between "open cell" and the fact that I am not even sure if the size of the stuff is right...

Any one have suggestions for a good site for portlight seals?

This is the "cut to length" stuff:

Open-cell foam rubber. Ten foot lengths.
3/8 inch tall, min. width 1/4 inch, max. width 7/16 inch.