Another option, suggested by camping guru Cliff Jacobsen, is to sew a vertical seam in your rear pocket. You can carry the knife vertically in the outer part of that pocket with great comfort while sitting. I do this with my old style slider cell phone (or with a large knife if not carrying the phone). With dress slacks, this would be invisible. I wear jeans 99% of the time and the seam is visible but so what.
A picture is worth 1000 words. Note the vertical seam in the center of the pocket. Note that it is shorter than the phone and knife. Makes for easier access while still keeping the phone/knife upright.
Untitled by
Pinnah, on Flickr