Once upon a time I came across a cheap, Chinese-made, lightweight nylon tri-fold wallet with a velcro closure and an external zippered coin compartment. It measures about 5" wide by 4" high closed. I currently carry a Large CRKT Eros in the coin compartment. It also easily accommodates a Small Sebenza or a 3" XM-18. I put a lanyard on the knife with a bead at the end of the lanyard. Then I extend the bead slightly above the zipper and close the zipper on it. From that point on, the knife all but disappears in my back pocket. Then all I have to do to extract the knife is remove the wallet, unzip the coin compartment, and pull the lanyard.
Even though the wallet was cheap, it shows no signs of tearing or falling apart after a year of constant use. I've searched high and low for it and I can't find it anywhere. If I ever do, I'll buy a box of them and put them up for sale in the gear and gadgets section. It is hands down the best wallet I've come across for carrying a lightweight folder regardless of price.