I just got hold of 2 Ion kite knives (one for my ProLimit and one for my girl's brand new ProLimit harness which didn't come with a knife).
I have a knife pocket on the back of the harness, but that wont hold a knife any longer (that's the reason I lost my previous one). The Velcro is a joke. One crash and it will slide out of there.
My girl's harness has a little pocket underneath the spreader bar, but it would be hard to pull out a knife if under power.
Do you have a string attached to your knife and have it somewhere else, easily reachable, safe but handy?