Sounds like a great project! Here are some storage options from Archival Methods, one of our partners (I also work there). For clippings, I like to use the Side-loading Print Sleeves with a cardstock insert. The insert provides support and a place to write any caption/identifying info. Plus you can get another clipping in the sleeve on the opposite side if needed.
For the sleeves, the size that will work for you is probably Item #32-112, 14″ x 18″, so a little larger than you wanted. The inserts are Item #79-1418 and the same size. You could cut both down to the size you need, of course.
Then I’d stack them in an archival drop front box, either 1-1/2″ or 3″ deep depending on how many you have. Item #01-116 is a 14-1/2 x 18-3/4 x 1-1/2 and comes in black. Also comes in tan or gray. Item #01-036 is 14-1/2 x 18-1/2 x 3″ in tan; also comes in gray.
Are you digitizing the clippings before storing them? How are you cataloging them – collecting the digital files and/or making a spreadsheet? You’ll want to translate that to the labeling of either the clippings in their individual sleeves or at least at the box level.
One more tip – the Forum is not as heavily trafficked as the members-only Facebook page. If you’re on that, it’s not a bad idea to post a link to your Forum post and ask people to head on over to answer your question!
Hope this helps with the newspaper storage. Even in archival storage, they’ll still deteriorate! You’re just slowing it down 🙂