Air Layer in a bottle
If you get in early before the leaves get too large, you can place a cut-off plastic bottle over the girdled branch as the tree begins to leaf. Fill it with potting mix and cover the sides and top with foil to keep the hot sun out and water every second week. The one in the picture was cut from the tree after 10 weeks and used a 1.5 litre (2 pint) soda water bottle. This provides a more physically stabe setup than a plastic bag but still has good visibility to check for root growth.
Here is a link that might be useful: