I have used foam board for the bottoms of my purses, I just cut a piece of the inside fabric big enough to fold over to the underside and I use the painters tape to tape it down. Once it is in the purse no one can see the bottom so I don't go to the trouble of making a "pillow case" for them. I also move them around since I have several purses from the same pattern - no one sees them so why waste the time making more bottoms.
I just saw a tip about using old cutting boards for purse bottoms - haven't tried it but I would think it would work.