A bunny rug is generally used for wrapping/swaddling, then another bunny rug or thicker blanket over the top. It all depends on how cold it is! I wouldn't worry about wrapping in a muslin wrap and then wrapping in an additional wrap. I'd just use one wrap, then additional blankets. The wrap is mainly used to help them feel tight and secure and to stop their reflex movements waking themselves up

Any thin blanket is great for wrapping - regardless of what it's called on the package - usually around 1m x 1m, some a little smaller/bigger. Thicker, bulkier blankets for over the top