These firm flexible plastic grid sheets are an essential part of any flat bottomed bag.  They are easily cut to fit your project and are available in 3 sizes.
 
Small - 13" x 10.25" (335mm x 260mm)
Medium - 18" x 12" (457mm x 305mm)
Emma Tote Bag/Emma Frame Bag - 11cm x 28.5cm - enough to make one tote bag from Emma's Tote Bag Stitch-a-long, details of which can be found here, or one frame bag from Emma's Frame Bag Stitch-a-long, details of which can be found here.
 
This product may be rolled for shipping.  Simply place a heavy object on the top to flatten it back out.