Here is an easy way to rig a backstrap. Take a belt that fits around your waist. Put two loops of rope through it that extend about 12 inches from the belt.
Place the belt around your back with the buckle facing away from your back and the loops extended.
Hook the loops onto your loom bar. You will have to experiment a little bit to get the right loom length. It's not fancy but should hold up until you weave a fancier backstrap.
Other ideas for making the backstrap include some that have been mentioned on this thread already.
- Cut off legs from a pair of jeans for improvised backstrap. I thought about that one because I save old jeans for the purpose of rag strips for making rag rugs. You can also convert the "shorts" into a shoulder bag by sewing the bottoms/legs closed and using some fabric of the legs for sewing the shoulder strap.
- Take a dishtowel, tie a knot in opposite corners so you can attach a rope. You can also take an old terry cloth bath towel and do the same thing. You might have to rip it in half to reduce the bulk.
- A pillow case can be used with knots in the corners.
- In the Curious Weaver Journal #2 article they suggest using a length of rope around your body as a strap while you weave yourself a strap as a first project. You may want to pad that rope with a towel or something.
- Old sheets can be ripped down to a wide sash sized strip, a knot in the ends, attach a rope.I hope that helps.
Have a good day!