Decided to try and beat your ship for fun, and made this... monstrosity:
It worked, though not well, and its gotta be the ugliest thing I've made in a while. I could've improved it a lot, but for a ship I threw together (and it was well under 1 million, to boot). The only problem was when my ship would split yours into pieces, which would cause mine to be threatened if it got too close to these fragments. Other than that, it simply hovered outside range and slowly but surely ate away at yours.
Also improved my other wall design a little bit, still haven't tried the large cannons though.
EDIT 2: Large cannon variant
I've noted that the ship tends to be split apart when a module's reactors are destroyed, so I'll work on a fix for that at some point. I'd also love to find a way to add more point defense, to help against missiles.