Find reading a map at 50 mph a little difficult, not to mention dangerous? Tired of missing an important call because your phone's in your jacket pocket and you can't hear it at speed?
Welcome to $3.00 Tankbagsville, population you.
Head to you local hardware store, navigate to the fastener aisle (screws, nails, etc.) and look for the magnet section (it'll be small, just like the number of geniuses looking to make this tank bag). Then snag yourself four nice-sized, round magnets (1.25 to 1.75 inches in diameter) - ceramic or chrome plated, it makes no difference. You shouldn't have to pay much more than a buck apiece for these (cheapest in a 4-pack).
Then grab a one-quart (best for cell phones) or one-gallon (best for maps) Zip-lock style plastic freezer bag from either your kitchen pantry (if you're lucky) or your local grocery store. These are less than fifteen cents each when you buy a box.
Lay the bag on your bike's fuel tank with the opening facing forward. Now slide the magnets into the bag's corners and position the bag as you see fit. You may even wish to tape the magnets into place, but it's really not necessary.
Viola! You've just constructed a very capable tankbag. Have no fear - with quality 1.25" to 1.75" diameter magnets there's absolutely no way the bag (and your map or cell phone) is coming off your tank until you yank it off - even if you ride 100 mph into a hurricane. And when not in use, the bag will roll up to fit in a small pocket or virtually anywhere. Best of all it costs only only $3.00... Or less.
You can thank me later.

I’m gonna thank you now, if you have any more of these cheap bike hacks keep ‘me coming!