I use a strong cardboard wrapped in matching or coordinating fabric, slip stitch the end closed, THEN::::
I add velcro to both the bottom of the back and the 'other' side of it to the bottom of the cardboard covered bottom and secure it in place.
I do this to help the bottoom in place, There is nothing more aggravating . to me. than to have everything small roll up under the bottom of the bag when it is not seured in place.
Hope this makes sense!