As per Halitech's tip, the back piece should be as high as the upper end of the knife, to protect your shirt, skin etc.. Other than that, it looks pretty good to me!
A marlinspike knife is heavy for it's size, with the wide blade and marlinspike, so it should fit fairly tight to keep it from bouncing out. When is the last time you actually used a marlinspike? It's been a few decades for me! I used to have that knife, but haven't seen it for...well, decades.