I went to the picture framing department and bought the stiff foam board they use for framing pictures.
I use a spare rotary cutter and mat to cut it.
It gives just enough stiffness to the bottom of the purse.
I make a sleeve for it so it can be removed when washing the purse.
I bought mine at Hobby Lobby, the cost was 99 cents.
Don't remember the size but I have made six Bow Tuck purses so far and still have enough for at least four or five more.
Blessings,
Sandi