I'd wear it often while bike riding so in addition to being reflective, tough, and waterproof, it would ideally be very lightweight.
I went to Paron Fabrics in the fashion district in Manhattan and bought some waterproof material intended for rainjackets. However, unlike many thinner rainwear fabrics this material is very tough and totally impermeable.
I added some high-vis and reftroflective elastic pieces to the outside back. I also made the shoulder straps out of plastic band with a built-in retroflective stripe.
The end result is a bag that meets all my initial criteria, for ~$20 in materials, and which shows bright colors and reflectivity from front, side, and back.
And of course with the flash on, the retroflective strips jump out. I made the design fairly randomly but when I finished I was reminded of the hanzi 火 which means fire. (Pronounced huǒ in Mandarin.) So I think the next bag will have a big 火 on the back.
The exact width of the backpack was predicated on containing the notebook I made. And the two worked together quite nicely...until the heavy, tough notebook proved so heavy and tough that it slowly tore the bag open!
Still, it had a good run. Either the next backpack will be tougher, or the next notebook less aggressive, or both. ⁂
The predecessor to this backpack was one I made out of some great material I found on the street in Manhattan: a big USPS bulk mail carrying bag.
Not one to turn down some great, tough material, I made a simple, skinny backpack that was wide enough to accommodate my (store-bought) standard composition notebook. Those aren't very wide at all so the backpack was pretty skinny and tall. If you squint you can see "MAIL" written across the middle of the bag.
I probably overdid the attachments from the straps to the bag, but I wore this bag for a year or more so it took a lot of abuse.
Size comparison—the old backpack was only around 8" wide and a lot shorter, as you can see. Good for packing light!
Further backpackery forthcoming: I made a backpack with/for my brother that I want to post here. Stay tuned.