I have found that you iron on two or more layers of a heavy interfacing on the (wrong side) bottom of either the outer fabric or lining makes a firm bottom for a purse. I don't like pillowcase bottoms because mine always seem to come up and the junk in the bottom of my purse hides under them. If I need a firmer bottom for a tote, I finish the bottom of the bag, then sew a piece of lining fabric over the bottom (on the inside), then just slide a piece of plastic into it.