I'm about your size and have a lot of trouble keeping guys from forcing over the unsecured leg - the guys are 100% right about the squeeze, hugging that knee and getting that opposite leg tight on their ribs, but while you're working on that you can try using that weakness and a little hip flexability as bait for the Carni: From Chill Dog you can sit far out on the off side hip (which will tighten your leg around their shoulder) and slip your attacking foot behind the knee of the free leg. From there he can pull back and lead you into an Omaplata, or force his way foreward (and usually over the free leg) right into the Carni.
Haven't seen an effective counter to it yet (I'm sure Brent's working on something...) aside from freak core strength, and once guys realize you can crank out their arm they'll have to give you more time in Chill Dog to work the position.