For progression just create more distance from of the bend fixing point
Find something realy firm and stable to connect the bend. There only one rule, the object that you fix the bend to, should be strong enough to keep the bend on high tension, a pole or strong tree will be the best. You do not want that piece of iron or other hard thing will meet your face on a high speed. The list of good things you want in your face is really short, a flowers or feathers on high speed is ok, but a door handle in your face is very very bad, and dangerous.
You can use more than one band to make the exercise most comfortable.