Wow... That is one awesome bag. It is totally worth the $50, if you ask me. I'd pay twice that amount in the blink of an eye. I don't mean to discourage you from taking on the challenge, but you might be better off buying it. We're talking major industrial sewing machines used for sewing specific parts of the bag.
Canvas is usually 100% cotton. Cotton shrinks. As with all your cotton fabrics, you should wash it, dry it in the dryer and then iron it. That way, should you decide to wash (or iron) the final product, it won't warp on you and lose it's shape due to shrinkage. I have worked with canvas a lot and I always treat it like that before making anything. I am able to wash it and dry it and iron it with no problems.
I find that most of the time it is more cost effective to make your own purses.The feeling is wonderful when someone compliments your work. That is mostly the reason I make them. I must admit that I do still give in to the designer handbags from time to time and spend a pretty penny on them. My husband also likes to buy them for me as gifts so I have a nice collection of designer handbags. But my favorite ones are those I designed and made myself. I carry them with pride and if anyone asks where I got the purse, I gladly tell them that I made it and that I design and make handbags myself. So my advice to you is GO FOR IT! :-) Try it out. Make a couple, see how that goes. I almost guarantee you will see that it is worth your time and money. Even if it costs more than buying one in the store.
I must admit my choice of fabrics were rather difficult to work with because of the thickness. I used a faux upholstery weight suede and a cotton twill. However the difficult part was that I used sew-in interfacing and my seams were really thick (HTC Ultrafirm) . I padded the laptop compartment with HTC fleece interfacing and sandwiching it all together made for a very thick seam. I had to loosen my dog tension and my thread tension and use a heavy duty needle.
Oh Dear... If it's Pellon Craft fuse, I can relate. It is really bad. I have used it a thousand and one times and it fails me everytime. I keep using it to see if maybe it works as well as others say it does but of course it never works. Also because I bought quite a bit of it when I first started sewing and I don't just want to throw it away. I use Peltex, it is made by Pellon as well, but it is a thousand times better. It is much much sturdier than the craft fuse but I don't know if it is available in fusible form, I've never seen any. I've always bought it in sew-in form. That will surely work for you.
I have tried so many combinations of interfacings (one for lining and one for outer fabric) and I came to the conclusion that a light weight fusible interfacing is always best for the lining, it makes it stronger and less likley to rip from carrying things like pens and keys. For the outer "shell" I always use a sturdier interfacing so that it has body and holds it's shape. This way I have a bag that will last and look good. Thanks for the compliment on my bags. I have not posted my latest projects using the fleece, but I hope to do so soon. I hope this helps .
Try a product called Liquid Stitch. It is a fabric glue. I also second the recommendation of the Stitch Witchery. I use it all the time for various applications in sewing and it comes in different strengths. I like to use super. It works out really well for me. Check out my recommendation titled "Iron-on Bonding" in the following link to see how I use it.
My guess is that it will be a challenge but probably not impossible. You might have to leave a very large opening in order to pull it through. Your best bet is to do wrong sides together, turn in the edges and tops stitch along the top. That way you do not mess with the integrity of the materials. I occasionally use sew-in fleece interfacing and it is a real chore to turn and knowing how stiff the peltex is, I highly recommend doing this way, I hope I am not too confusing. Let us know how it works out for you and possibly post your finished product :-)
With sew in interfacing you would still have to have the interfacing the same size as the pattern pieces so that you can sew it to your fabric. You need to sew them together close to the edge so that when you sew your pieces together with your regular seam allowance the stitching from putting the fabric and interfacing together does not show. So basically using sew-in interfacing is no easier. I'm sure your machine can hadle it. I have sewn some of the thickest interfacing in that manner many times and my sewing machine is just fine. I have a regular sewing machine, nothing industrial.