I find the small plastic bags used by people who ship small collectibles like Legos are ideal for bagging small objects. I use it to keep little camp rolls of duct tape from getting adhesive on things, and I wrap lighters, tools, memory cards, and batteries in them too. Freezer bags for your DSLR are a must! Good call.

I worry about plastic pieces. I've broken plastic pedals, plastic water bottle mounts. I've broken plastic light mounts. it's that one fall, or that one time you accidentally put pressure on it, or the one rock that flicks up, and you regret using plastic. I defer to flexible materials like cloth all the time. In my head, I see a bracket that holds a handlebar bag flexing a centimeter or two every time you go over a bump over and over and over, and I have a hard time trusting it. Experience from someone with thousands of miles is a big plus, but it still makes me nervous with a DSLR. If I have four attachment points with rope or velcro, the chances of all 4 breaking simultaneously are outrageous.

Of course, I could always find a plastic mount bag and then tie some velcro on as a backup!