So...yes, of course there are, but you're fixing a different problem than I'm trying to -- my biggest concern is sound leakage outwards to my neighbors, rather than inwards into the microphone. So something like this would be the pre-built alternative:
https://isovoxbooth.com
https://whisperroom.com/application/voice-over/
https://www.clearsonic.com/collections/megapac
I was thinking more along the lines of a wall rack like a vertical futon holder like Naaman described, I was just trying to think of a way to utilize what I had before resorting to the prebuilt option, since I already had the mattresses and they aren't being used.
And Jeff -- you're right of course, I would like to eventually record vocals professionally, but I'm less worried about that for now -- mostly I just want a place to practice in, late at night, and record for my own review.