I used to use plastic wallets with a plastic zip close, like a beefed-up "Ziplock" bag, from office stationery suppliers, but I found that the plastic was not flexible enough and would crack after a few weeks of weekly travel, usually while away - so even though they are cheap, they are too cheap.
I now use
this bag from Eagle Creek which is made of a more flexible plastic, and so far no airport security has complained about it and it is showing no signs of wear after some months.