Boxing the Corners
If you've made a flat-sided tote bag, but you want to give it a bit more depth, you can box the corners. It's easy with square corners, but you can do it with a rounded bag, too.
Boxed corners: Stitch across the corner.
Sew the point to the bottom seam.
Turn the finished bag inside out. Flatten a corner so the side seam is in line with the bottom seam; the point where they meet is at the point of your fold. Determine the amount of depth you want. Sew across the corner, perpendicular to the seams, as shown in FIGURE 10-2.
Fold this tuck down against the bottom seam, and hand-stitch it to the seam allowance with several large looping stitches, as shown in FIGURE 10-3.
Once this is done, you are ready to reinforce the bottom. Measure across your boxing stitches and from one line of stitches to the other. Cut your fabric to these dimensions plus 1″. Turn the edges under ½″ and sew this piece to the inside of the bottom. You will probably have to sew this entirely by hand. Depending on the size and flexibility of your bag, it could be difficult to get to it with the sewing machine.
For a tuck in a rounded bag, turn the bag inside out and take a small tuck in each side, near the bottom of the downward curve. Pin the tucks and turn the bag right side out to see if it gives you the effect you want. You might try different sized tucks at different locations. When you find the perfect spot, fold the seam so you can sew along the tuck in the seam. You will stitch just a few stitches on either side of the seam rather than stitching clear to a fold as you would with a square corner. Be sure to backstitch so the stitches don't pull out.