We once made our own anti-creep device on a two lever system that worked quite well.
Take a short(2") piece of rubber hose (I.D. about equal to the outer cable cover O.D) and slit it lengthwise. Where the cable connects to the mechanism inside, clamp the hose tightly to the cover, overlapping the exposed cable end that moves. Then apply a small hose clamp over that end and adjust the tension so that you can still adjust the throttle but it provides enough friction to eliminate creep.
Costs next to nothing, is out of sight, ours worked for years ( and may still be doing so - we sold the boat)