I've been meaning to update this bag base tutorial for a while. Not only was the original hastily put together from a black & white pdf document (long story involving band width), but I've ever-so-slightly changed my method since it was written. (And colour is always nicer than grey, don't you think?).
This tutorial is for a base in any boxed corner bag (and it can be adapted for other bag constructions).
1. Measure the length of the base seam (above) and then the boxed corner seam (below).
2. Round both measurements down by around 6mm (1/4 inch). For example, on the boxed corner seam below, I'd round down to 6.5cm.
3. Cut a piece of Peltex (it doesn't matter if it's single-sided or double-sided fusible) or heavy weight Fast2Fuse to the (rounded down) dimensions of the base seams. (I find that a rotary cutter and quilting ruler are the business for this job).
4. Roughly cut a scrap of fusible woven interfacing large enough to wrap about 2 and a half times around the base piece - folding along the long edge. You'll also need a generous seam allowance (overhanging the Peltex/Fast2Fuse) at each of the short ends. No need to worry about accuracy here, folks! 
5. Wrap the interfacing around the base piece and fuse it all together with a hot iron. The interfacing will stiffen as it cools. You can add further layers of interfacing if you'd like to make the base more rigid.
NOTE: If you use double-sided fusible Peltex or Fast2Fuse, you can use calico or fabric scraps instead of interfacing. (That's what I used to do before I discovered the benefits of layering up fusible interfacing).
Have you tried Peltex or Fast2Fuse as a base? You can find it all in the shop.
More free patterns, tutorials and sewing tips, are over here....