i'm trying to wear my shirt as a crop top without cutting but the back keeps falling back down, i'm using a rubber band to hold it up. any help?

i take the back of the shirt and roll it up into a knot and then after rolling it up a bit i put the rubber band on it. this works fine for the most part when i sit down but when i walk around the back of the shirt likes to fall back down. i'm not cutting another shirt. plus i don't like the look of a cut shirt anyway. any advice?