saw mudflap made from innertubes https://youtu.be/isbiYv7wEps

I'm a retired professional kayak fisherman and I own this piece of gear that I bought a few years ago. It is called "Leg luggage"http://www.legluggage.com/  it is a neoprene wrap for your leg and you keep important gear handy attatched to this sleeve wrap using velcro pockets. I took it one step further and took three of my gear bags-one with a hammock, one with a bug screen, and one with my tarp. I put the three bags together and wrapped them into my neoprene leg wrap which has velcro closures that attatches to itself. Then I use velcro strips to wrap around my handle bars and attached it to neoprene wrap, holds like a charm. See Photos. Also you could probably use one of those stretchy neoprene back supporter http://bit.ly/2pxJlJn as well. Then attach it to your handle bars with some 1" Sleeping bag straps http://amzn.to/2qcCvFy

they make nicer bags if you are willing to pay a few dollars more. here is one with a zippered top http://amzn.to/2pwR6PE

I bought another type a little heavier duty material made from Codura Nylon I am building an underseat bag, will post photos when I get to that project.

I found something unrelated to biking that In a short bit of time I could convert into a inexpensive frame bag.

Here is what I did to make this bag.

I picked up a set of these(4)Canopy Sand Bags http://amzn.to/2p1jJ68 they have a set of velcro straps(4) already attached for the upper tubing of the bike.
I use the velcro to attach to the top rail of the bike. I cut the top webbing strap about a foot with about 6" left over on one side, then I put a 1" plastic buckle on the webbing I sew the buckle in place, the buckle i'm using has a built in flint and strikerhttp://amzn.to/2p1xwKd to start a fire if I need to in an emergency.