Epi speedys are built with a reinforcement strip along the bottom, so that prevents a lot of the sag, but I put a piece of cardboard in mine, anyway. I made sure to get the kind that you'd find on the back of a thick tablet of paper or something, so you know it's the heavy duty kind. The piece should be pretty unable to bend without breaking the cardboard. It shouldn't damage the bag, either. The corners are sharp but not enough to pierce the canvas by any means- the cardboard would bend first!