Doug, glad you are not the only one who has ever thought of this!

Here is my idea from sometime ago. Get a pool noodle (3.5" diameter size) from the Dollar store, Canadian Tire, Walmart etc. Cut a section length to fit the knife then cut the noodle in half lengthwise then trim to your needs. You could also cut a slit in the side of the noodle for an improvised belt loop attachment point.

Slip the knife into the noodle then press in so the handle is also in the noodle. The fit is so tight that you do not have to worry about the knife slipping out. Another good factor is the if you drop the knife into the water, it will float.