Hi caravansue, welcome to the forums. Scales might be a bit of an issue for you so why not just use a tape measure while you are travelling? It will certainly tell you a good story!
I would suggest keeping a log (paper or electronic) recording your blood sugars ( if you test them) and your measurements – taken from waist, hips chest etc. Each time you are in a larger settlement you may find a pharmacy with scales you can use and then record your weight.
If you don’t have a tape measure with you the string is an excellent idea, and when you get a tape measure you can get your original measurements from the string! π