I use a rope strap around the tree with a prussic knot on it . This way I can adjust the length for times like you just described . I keep it tight when sitting so I wont fall far but can lengthen at times when nessary . It can double ad a line mans belt.

The One I use came with my harness it's a seat of the pants. I also picked up a couple that I use as line mans belts from here
http://safetreehunt.com/
Very good quality and at. Good price.