Does anybody has screen shots fo actual fills for both legs? or it is just a theory!You can't buy BTC on CME, only futures. I'm pretty sure they're cash settled. Bitcoin only exists in the conventional markets as custodial fund instruments like Grayscale. You can't trade it directly on the CME. But I'm not expert on the latest developments, so I'm not sure what's possible and not now.
You can do both underlying and future - the entire trade - on Binance, Deribit, Bitmex or ftx. Others, too, but those are well-vetted exchanges with solid security. Some people don't like Bitmex because they think the exchange gives back-door kickbacks to market makers. I don't know if it's true or not.
ftx has the best spreads. Deribit has the best margin structure. Binance has the best liquidity, but I don't know about it's US citizen policy.
so if what you say is true then the carry type trade like this is via those 4 "exchanges" with their own spot BTC and BTC futures! and these exchanges do give margin offset? .. h...m I still think there is risk like SIG mentioned below. so it is not really a risk less Carry trade !