There is nothing about a "contrivance" that requires it to be made only by yourself.
Yeah, but here's what it means:
con⋅trive /kənˈtraɪv/ Show Spelled Pronunciation [kuhn-trahyv] Show IPA Pronunciation
verb, -trived, -triv⋅ing.
–verb (used with object) 1. to plan with ingenuity; devise; invent: The author contrived a clever plot.
2. to bring about or effect by a plan, scheme, or the like; manage: He contrived to gain their votes.
3. to plot (evil, treachery, etc.).
–verb (used without object) 4. to form designs; plan.
5. to plot.
That means that a suppressed barrel would not qualify, because that is it's design.
In layman terms, Contrived means "homemade" or "rigged". A coke bottle wasn't designed to suppress the noise of a firearm, so it is a contrivance. Basically, you can't wrap a gun in a wet towel like in Godfather 2 because that would be a "contrivance" to suppress the sound of the shot. A device would be something that suppressed the shot by initial design.
However, since there is no exception for law enforcement and several WA state law enforcement agencies use them at public ranges, it would be really hard for any court to get a conviction, which is probably why there has never been one.