Almost every bag/purse I make I put something in the bottom to make it stiffer so the bag will not sag and to give the items I carry some support. Most of the time I make the bottoms the same way, using the same materials.
I like to use plastic needle point canvas and cut it the width of the purse/bag bottom and the length of the bottom of the bag. This messenger bag I just made has a curved bottom so just cut the plastic canvas to the length just as the curve started upward. I also always cut off the 4 corners of the plastic at an angle so the corners of the plastic will not wear out the corners of the bag – that will happen over time if there are sharp corners. I probably cut a couple of the mesh off, about 1/4" from each side of the corner. If you feel you need more strength and more stiffness just stack up two layers and zig zag them together. This plastic canvas is very easy to sew through with a sewing machine.
I like to tack the canvas to the bottom of the bag before I add the lining. Since I am not fond of doing it by hand I do it with the machine. For this bag I zig zagged one inch strips to the sides of the canvas placing most of the strip off the edge of the canvas.
I pinned the fabric strip to the bottom seams of the bag and stitched the two together. The bag bottom will now stay in place and the bottom of the bag will be stiff enough to hold what I want to hold. I want this bag to hold my camera at times when I don’t want to carry my Canon 2ti DSLR camera in the camera bag.
I have heard of using special bag bottoms you can purchase made of solid plastic or a Masonite type wooden product. I don’t like the idea of these as they will add weight to the bags and I want to keep my bags/purses as light as possible. They come only in standard sizes so you are limited to those sizes for the bags you make. I also can purchase the plastic needle point canvas and cut it to what ever size I need – can keep it on hand so it is available when ever I need it for a new bag. It has worked well for me so will not change unless of course something better comes along.