Reddit mentions of Flexible Plastic Cutting Board Mats set, Colorful Kitchen Cutting Board Set of 3 Colored Mats

I think kydex might be a good choice. It is pretty cheap, and also heat moldable. Just make sure to check the thickness of the sheets before you buy. If they are too thick, they won't be very flexible. The .060 inch thickness is pretty stiff, but still bendable by hand. You can pop it in the oven and then lay it on a cylindrical object to achieve the desired curve, and then use a heat gun (or a candle, if you're feeling cheap) if you want a more custom fit.

https://www.amazon.com/Kydex-Plastic-Sheets-Black-0-060/dp/B00HAS1VSK/ref=sr_1_4?s=industrial&ie=UTF8&qid=1495674072&sr=1-4&keywords=kydex

Otherwise, I have also had luck with those flexible plastic cutting boards, like these:

https://www.amazon.com/Flexible-Plastic-Cutting-Colorful-Kitchen/dp/B01HN7ZGUQ/ref=sr_1_2?s=industrial&ie=UTF8&qid=1495674423&sr=1-2&keywords=flexible+cutting+boards

They are pretty good for making crud-catcher type fenders, with liberal use of zip-ties, of course.

Good luck!

You won't find a concrete set of rules for it because there really aren't any; how blue the surface would look would depend on how close, how bright, and how focused the light is.

If you really want to learn this stuff, do lots and lots of studies from references and from life. Get a little light like this set with colored inserts, shine a bright light through a diffuser (even a homemade one; these flexible cutting mats make great colored diffusers, or you can use a white one for more natural color).

File holders may be cheaper. I used these:

https://www.amazon.com/Flexible-Plastic-Cutting-Colorful-Kitchen/dp/B01HN7ZGUQ/ref=sr_1_4?keywords=flexible+cutting+board&qid=1570654834&sr=8-4

So have enough to last a lifetime now. (basically used 2"s across the short side of one)

Some context on the pocket. We travel a lot to various places and often find ourselves exploring a city, finding ourselves in crowds and whatnot. It is nice having a light pack to bring on the town with a jacket, etc, so that's kind of what this pack is aimed at. And when in those crowds it is nice to have the peace of mind that nobody can get into your pack. So a pocket against your back fits that nicely. (as does a roll top fwiw)

Stitching is still terrible, but definitely getting better. A lot of it is learning the idiosyncrasies of the machine. For example the thread I have has a tendency to get bound on the pin on one of my pins and I don't really notice it right away so that causes high tension and everything goes South. Lots of variations on that (threading and getting a loop around the screw of the foot retainer, not holding the thread as I start sewing, not keeping pin down when stopping or rotating etc..). I'm getting better at checking all those things and also getting better at controlling speed, so confident my next will be worlds better but still a lot of things are goofy on this one.

I did do the bathtub again, but ya, this time didn't cut at all. I'd say it is a bit trickier because lining up the bottom isn't quite as obvious. You also end up with an odd flap there. I ended up folding that flap into itself, then sewing it through on the open edge and that makes it pretty small and much neater looking. I think I like the bathtub setup for a small pack like this but would go for a single front panel for a bigger / more serious one.

One question for anybody listening, on binding with grosgrain (which I haven't done yet), what is the order of operation there? Do you put in your "real" stitch at say 1/2", then go back with the grosgrain at 3/8"? That seems to be how packs I have are done but not sure on best practices.