I'm a big fan of surplus stores too. A few years back I was in one and saw a couple M40/M42 gas mask bags. I think they were six bucks each. I took them home, did some cutting and resewing (with a needle and thread, I don't have a sewing machine), and hung them on the crash bars of my V-Strom. They're great bags; real heavy duty nylon and canvas webbing, full length velcro closure for the flap, they even had a little external pouch (originally for a decontamination kit).
I traded the V-Strom in 2014 and got a Yamaha Super Tenere. The crash bar bags got remade then. I got rid of the big single pull strap for the flap, and the external pouch, and replaced the pull strap with a couple polypropylene webbing straps with quick release buckles. The old attachment points had to be removed since they didn't fit my new crash bars, so I made new ones out of that velcro tape you find at a hardware store. The bags were sprayed with black Krylon Fusion spray paint, which sticks pretty well (but not perfectly) to plastics. The finishing touch was an embroidered patch of the Taureg Man, which I sewed to the flap.
Eventually, I remade the bags one last time. Because the radiator on the S10 is on the left side of the bike, the position of the left side bag blocked too much of the airflow and made the bike run a little hot in stop and go traffic. I scrubbed all the black paint off (washed it on heavy cycle a bunch of times in the washing machine) and repositioned the attachment straps to fit the new bag position lower on the crashbars. The last photo is how the bags look now.
I love these things. They are tough as nails, they were cheap, and the modifications weren't difficult. They're perfect for carrying my and my passenger's rain gear so it doesn't take up room in the panniers.
"No matter where you go, there you are."