If you are going to actually drive the car much, it is difficult to get the strap just right so that the window is up tight and you can still snap it. Also, even the tough seat belts can stretch just a bit over time allowing the window to drop a little bit.

I dealt with this by using clean used seat belts from the junk yard and attaching the window end of the belt to a loop of stiff elastic band. That way I had some give on the strap so that I could snap it easier when the window was up, and the window was always pulled up tight to the weatherstrip.