First up: a barrel band is not intended or designed to reinforce a stock/forend. It is to (mostly) distribute the load of a sling attachment or be the point of a bipod attachment. If you make a barrel band for such a purpose it will be quite robust and out-of-place looking. That said, however, if I was making one, I would take something like very bendable, like soft solder, and wrap one layer tightly around the area in question. Cut it so that the two ends of the solder butt closely to each other. Then straighten out the solder and measure, with a decimal scale. Next divide that dimension by 3.14 and you will have a number which would be the diameter of a piece of tubing you could use to make the real band from. Next you would need to be very patient in slowly shaping the piece of tubing ( which was cut about 1/4 inch wide) to fit both the barrel and stock, with them both assembled. Once it was tightly fitted and cleaned up, it could be finished, somehow.
But the finished product will have all the marks of an amateur job, and won't really be reinforcing the stock to prevent future cracking. And anyone who saw it would not be impressed, in my opinion. Much better to get or make new wood.