Inventions like these always tickle me. These ZaVarge Bedding Clips were designed to keep the corners of a comforter aligned inside the corners of a duvet cover.
Installing them is a bit of an involved process:
I’m not sure I’d want to pierce a down comforter with an awl, and these do seem like a long way to go to solve the problem of a wandering comforter, but I certainly don’t have any better ideas.
No link to buy, sorry; at press time the company’s website was dead, so they’re either a COVID casualty or their server’s down.
Source: core77