I have noted a number of posts that remarked about the screens not fitting particularly well on the portholes. My 86 31' has couple of screens whose frames have become brittle and all but given up. Likewise, some of the knobs have cracked and won't grip the post when turning. Even the interior clear plastic windows are showing cracks. I could replace the whole porthole, which seems pramature since they don't leak when tightened. I could replace the knobs, or try some way to band and strenthen them. Has anyone found a source for the screens, or fabricated an improvement.