This is borderline ASB but:
Fight the War of 1812 a bit earlier.
Have Nappy do a bit better the first time around.
Brits settle the War of1912 to US liking in about 1813
Settle with Nappy about then
Blame US for losses...Stab in Back by $%^&% cousins
Find sources of cotton and tobacco outside US: Egypt?
Europe uses those sources also to drive down US crop values
Slave economy collapses.
I think Britain would be pretty angry about a loss to Napoleon, but the War of 1812 was more or less a sideshow to them--I don't know if they'd focus their ire on the USA unless either:
A) America hands them their asses in a major, resounding defeat that contributes to said loss to Napoleon (which is unlikely in the extreme), or
B) The War of 1812 devolves into a bloody stalemate that draws off British resources and manpower, essentially becoming Britain's Vietnam.
That said, if Option B happens, there are plenty of opportunities to screw American slavery outside of "fuck them, we'll buy our cotton somewhere else"--the Brits could encourage slave uprisings; the South could become a major theater of war and be devastated in the fighting, destroying many plantations; a British blockade could prevent cotton sales to anyone, not just Britain itself; and so on.
But the downside to all this is that the longer the War of 1812 goes on, the higher the risk of a New England secession gets, and if that happens the US is lost to the slaveholders forever. You'd need some kind of POD that makes the US either stronger as a whole, more anti-British, or both.