You know, I really wish I had read this thread before you did all this work on the gun turrets. I actually know of a part which would be perfect for laser cannon barrels:
See the little part on the left? Looks like a laser cannon barrel, doesn't it? You can find them in disposable lighters, and this one was from a Bic lighter. They look like this:
Anyway, if you take it apart you'll find an assembly on the inside which can be disassembled to reveal several small parts, among them a little nozzle which can be used to make a pretty cool cannon barrel. You'll probably want to get a similar brand for all the turrets, as the design of the nozzle can vary from lighter to lighter. I found a deal on ten of them from Walgreens for $5 total, which isn't too bad for such a cool little part. And as you're using pop-rivets for the barrels and I know what they are, I can tell you that these are about the same size and could be used if you want to replace the barrels with something more turret-like.