I too like flat bottoms. The bottom of my bags measured approx.16" x 5". I cut a piece of the fabric to place on the bottom approx. 17" x 6" and sewed it centered on the bottom on only the long sides. The short end sides I left open to be able to slide in a piece of cardboard to help make the bottom stay flat. This will enable you to take out the cardboard when you want to wash it. LMK if you need more explanation. Hope this helps.