I think people overthink what's necessary for babies. I put mine on the floor; they would roll around, but never stayed very long in any one position. I got down there with them and just played, sang, talked.
So don't do it. I don't understand why people continue to do things like that. Just let the baby play. He or she will develop their core muscles without help. Holding your kid upright and having them lay on your chest and most every day activities will be good enough.