I've done the same thing as that guy, but with the splinter cannon as well for the third HWB (shaved a lot of the sticky-out bits off, and shortened the barrel) and I've been using the sheathed scourge knives minus handles for the thingies at the end of the barrel.
They're a bit more gaudy, but I think that helps, because they look really obviously the same kind of weapon and it stops you focusing on the differences between the guns.
Dark Angels. The Rock. Lion El "Dwayne" Jonson. I can't be the first person to have thought of this.