I started with an extra Assault on Black Reach Multi-Melta arm like this one. First thing to do is cut off the front weapon parts and the cables underneath. I also decided to remove the fuel canister tips from the back end.
I had to add a bit of plastic to the bottom cutout, as I hadn't accounted for the part where the wires enter the backpack. I cut the backpack part to fit in the bottom cutout, figured out how to make the original large cable fit the gap, and used a piece of electrical wire to replace the original smaller cable. I also added a few bits to break up the flatness of the back.
Like the sentinel conversion, I would probably have saved a lot of time by buying the Venerable Dreadnought Plasma Cannon arm bits. Whether I really did it because I enjoy converting, or just to avoid the paint pots, it was a fun diversion.