Yea what i've been doing lately is grabbing my pant sleeve with my left hand and there's plenty of pressure to force a tap there even without locking it behind my knee, but the worry is always what if I'm not wearing a gi, and also that leaves me with an arm tied down holding the lock (but maybe that's not a bad thing if the other guy is about to tap).
And honestly, losing weight won't really help me. My legs are all muscle. Big burly muscles. I'd have to lose that muscle in order to thin up enough to do a triangle without having to pull on my pant sleeve or leg