I use, ComeFront, as I back up three steps or so. Start with a treat or toy, and I use the word I use with recall, COME, and add the Front, so that he sits, directly in front. I suppose you could used ComeSit, kind of all as one word. It's so quick that when you say, Come, the dog is already turning and coming toward you, and then sit.
I have trained this as a stationary sign, I think there is this in Rally advanced. That might be easier for you. With a treat or toy, while you are standing still, move it out in front of you, and say, FRONT or FrontSit -- Double-commands are not penalized in rally, but do-overs are. Using FrontSit as your command or ComeSit. Is not really a double command anyway. But you lure the dog out and turn him in front of you and finish in the sit position, close enough to touch the collar or pet the head, and praise and treat if you are using them.
Heidi Ho, Odie
Joy-Joy, Bear Cub, Hepsi-Pepsi
Cujo2, Karma Chameleon
Ramona the Pest, Kojak -- who loves you baby?
Tiny Tinnie, Susie's Uzzi, Kaiah -- The Baby Monster.