Having raided my bits box, I finally selected a Melta Cannon for the previously mentioned Mark II "milk float" Warlord.
The Melta Cannon is fairly plain, but the version created for the Forge World Reaver Titan looks rather cool, so I thought I'd have a go at improving the stock version.
Apart from the arm, which I made from plastic rods of various sizes, there are three new additions. First is the piping on the back - this is from the arm of a 40K scale Necron Destroyer (the arm is discarded when you upgrade to a Heavy Destroyer). Second, I've added the muzzles from a Necron Immortal's Gauss Blaster to the front of the gun.
Finally, the underslung piping and housing is from the powerfist of a Space Marine Terminator.
Here's the Melta Cannon, attached to the main body of the Titan: