I have been searching for this for a year. Forum discussion usually come out to “why would you need that?” I work in media and have to look at an enormous amount of video and photo content, quick look is an absolute necessity!
It looks like someone already stuffed it with insulation.
Assuming this is in an unseen location, you don’t have to get fancy. You could put a square of plywood over the hole, screw it into place, then caulk the edges. That should cut down on drafts and keep creepy crawlies out.