2 Options that might work.
First, like dumpsterDiver said
Tie a shock cord loop on the lanyard handle and loop the resulting loop over the sheath of your knife when not in use. The tension will make sure that the knife doesn't come out until you slip your hand throught he loop and pull out your knife. The benefit to this is that your hand is immedietly on the knife and attached through the lanyard. So no fumbling to unhook the lanyard from your BC then unhook the knife etc when you're in an entangled situation.
For this you'll have to keep the sheath braced on something like your leg or attached to the shoulder strap (not the excess that will flop loosely)
Second option is to make your own sheath out of 2in nylon webbing. Fold a piece of webbing as a Z, leaving one side shorter. Stitch it up through all layers in a U shape so your get a sheath with a belt loop. This creates a friction sheath which is the best type of sheath I've used. The only issue would be if your knife fits it well. I'm unsure of the width of your knife handle. But depending, this option might not be best. You'd also need an industrial sewing machine or a cobbler (shoe maker/repair-er) who can follow directions. (I've had mixed luck in that department)
For this sheath to work, you need to aim at making the belt loop 2 in, and attaching it to any 2 in webbing. It isn't so secure when attached to your leg.
First, like dumpsterDiver said
Tie a shock cord loop on the lanyard handle and loop the resulting loop over the sheath of your knife when not in use. The tension will make sure that the knife doesn't come out until you slip your hand throught he loop and pull out your knife. The benefit to this is that your hand is immedietly on the knife and attached through the lanyard. So no fumbling to unhook the lanyard from your BC then unhook the knife etc when you're in an entangled situation.
For this you'll have to keep the sheath braced on something like your leg or attached to the shoulder strap (not the excess that will flop loosely)
Second option is to make your own sheath out of 2in nylon webbing. Fold a piece of webbing as a Z, leaving one side shorter. Stitch it up through all layers in a U shape so your get a sheath with a belt loop. This creates a friction sheath which is the best type of sheath I've used. The only issue would be if your knife fits it well. I'm unsure of the width of your knife handle. But depending, this option might not be best. You'd also need an industrial sewing machine or a cobbler (shoe maker/repair-er) who can follow directions. (I've had mixed luck in that department)
For this sheath to work, you need to aim at making the belt loop 2 in, and attaching it to any 2 in webbing. It isn't so secure when attached to your leg.