Defender or Beckson:
Beckson Window Port Gasket GK-410, GK-512
I replaced gaskets and screens. I did do a self repair on a few of the screens as I had some screen left from doing a home screen repair. I used what I had but words of wisdom.....if the screen is too dense it keeps the bugs out but also the air is cut down. My boat harbors in wine country, near orchards, so keeping the fruit flies out in the fall is a bonus.
Have a hair dryer handy to heat the gasket and be patient. It WILL FIT,EVENTUALLY, but it will seem like they sent you the wrong size. Another thing, when u close the window for the 1st time, heat the gasket with the hair dryer, close+lock the window while the rubber is warm and leave it. The new gasket is not moooshed down like the old one and the window closing will be difficult. Warm the rubber!!!!
On a yearly basis I clean the rubber and treat it with gasket treatment. It comes from the auto store with an applicator similar to shoe polish....container with a sponge at the end. I also use it on my Outback doors& windows to keep the rubber pliable to cut down on noise...she is 11 yrs old and the rubber looks great.