Bifold Wallet
A bifold wallet designed for flexible filament. My version was printed in NinjaFlex SemiFlex, but others might work.
This thing functions just like a traditional bifold wallet and features a space for bills, four horizontal slots for credit cards (that can each hold at least two cards), and two vertical slots for scraps of paper or lesser-used cards.
EDIT 2-11-2016
Added a version with rounded corners so it isn't as sharp in your pocket.