You can go to a hardware store and have them cut a piece of acrylic for you. I'd make it about a quarter of an inch smaller in both dimensions than your purse bottom. Double check the measurements by cutting a piece of cardboard first. When you get the acrylic cut, take a piece of sandpaper and sand the sharp edges off the corners and all edges. Make a fabric sleeve for it and tack it in the bottom of your finished bag. This also works with all the other ideas above, plus foam core, corrugated plastic, etc. Another source for purse bottoms are the scrap bins at sign shops. Often free or minimal $$. I just score with a razor blade and the stuff usually just snaps.