Sleep Sack from Baby Surprise?

I've had an epiphany! My little sister is expecting her first baby and has asked me to make a snuggle sack for sleeping. I saw this post for a modified baby surprise jacket with legs, but really, I don't need to get that complicated. All I need to do is extend the body of the jacket down (so if you're looking at the front of a finished jacket, after you finish making right angles from the armpit, just extend the bit of back & forth knitting to make it a longer body). When finished, I just need to add buttons all down the front and across the bottom - or just down the front and sew the bottom together. VOILA! A snuggle sack. Even better - a stretchy, extra warm snuggle sack!

Now, to actually find time to do this.....