Just raid your linen closet plus a few binder clips" is close but not quite right, because binder clips alone will chew up your sheets and slip off the chair backs. The trick that actually holds is the cheap spring clamps from the hardware store, the ones with the wide rubber jaws, because they grip the fabric without tearing it and they clamp onto almost any chair or shelf edge. Binder clips are fine for thin stuff like a light blanket, but a real sheet pulls them right off in my experience. The $85 kits fail for the same reason, those tiny plastic clips just can't hold any real weight. So yes, skip the kit, but grab a bag of spring clamps for a few bucks and your fort will actually survive the cat.

Did you ever read that Kodak tech pub where they say fixer capacity depends on how much silver builds up, not just roll count? My local shop guy told me the clip test still works but only if you test right before you're about to dump it anyway.