From Problem Solvers sage customer serviceman Gage:

"You can buy cheap manageable sized tote-bags from thrift stores and easily convert them to stylish panniers,  I have done so using sheet PVC plastic and random hardware from the garage.

Thrift store tote bag...

 

Cutting PVC to fit inside the tote bag. This gives a stiff surface to hold the shape of the bag, as well as a strong enough surface for attaching hardware. 

 

Some random hardware bits from the garage. Ideally, something bendable. Think old rack struts, p-clamps, etc. 

 

Bend two straps in to "J" shapes

 

Attach the "J" shaped hooks to the PVC through the bag. An old inner tube makes a nice elastic strap to keep the bag secure to the rack. 

 

Inside the bag view; note hardware attachment.

 

The bag attached

 

Decoratin'. 

 

About 45 minutes later, you've got yourself a solid pannier for mostly free!