Yes, with that very heavy stuff, ,American cotton in 15 or 18 ounce, ordinary canvaswork with palm and needle is very , very difficult to say the least.
I have just made up a couple of pouches and your best bet is to use the saddle stitch wherever you can, and dont forget to use the saddle needles instead of the traditional ones of triangular section. Mark up with a very fine and hard pencil and pre-punch the holes with a round section awl.
For assembly and holding edges together, forget the use of tacking stitches or the sailmakers hook...get an ordinary stapler to set things up tight and neat and in very much less time. Just pull em out when youre done. Simple.
Dont forget to elastoplast each forefinger tip before you start, especially if you're not used to canvas work.
Make any creases nice and nippy by using a stroke or two of beeswax on the inside face, then really do a good job with the seam rubber.
Best of luck
Ceeg