When I saw our daughter trying to put on the Baby Björn I use to carry her, I figured I'd make a doll carrier for her. I wanted to make something simple and cheap, and easy for a little kid to work with, so I grabbed some leftover fabric and some velcro, and got to work.
This is a pretty simple project, I left it with one size for the arm straps, and one size for the pouch. It would be pretty easy to make it adjustable, by using a belt slider
to the arm straps or extra velcro on the main pouch.
If you go the extra-velcro route, however, be sure to extend the length of the soft (loops) side, not the rough (hooks) side. If you leave the rough side of the velcro open, it can snag on the doll's hair.