Here are some photos of what I came up with. Basically, I followed the pattern of GMs John Wayne movie photo. The style of it fits the quirkiness I was looking for.
Below: I started by cutting a wide strip of styrene gluing the ends together forming a circle. Not having tried this before, I was concerned with out of roundness. I was expecting I might need to add bracing bringing the walls into shape, but I didn't have to. I was able to bend and shape the styrene to form a decent circle. I was glad because bracing would have shown through the openings, I would not have liked that.
Below: You will see that I used these. Got them off Ebay.
I had a good day fabricating the guns yesterday. I didn't plan on making them as I did. Honestly, I had two goals in mind. I wanted two guns that were different in style and size, one of them having a large muzzle. The second goal was to add a little junk behind them to simulate a stock and/or whatever. It was a rare model session where everything clicked, and the guns formed almost on their own. I found pieces in my junk box that I could repurpose and that was half the battle.
Today-- things didn't go as well. I spent 2 hours trying to mount and fit the pieces. I needed four hands, 3 eyes, and a glue that set faster. Both Tamiya and CA were both painfully slow setting, adding to the struggle. But-- perseverance won the day.
For the larger muzzle I inset a piece of brass tubing to reduce the ID of the inner barrel. The styrene ID was a bit too large for my taste.
And btw. This is another example of Nemo's technology. These are magnetic rail guns. The nodes on top of the gun housings are electromagnets. They force projectiles out of the barrel at high velocity. 
The enclosure: The top rim was made using half round stock. The Bottom trim is T stock, similar to how I did the stack. The frame openings and top cover were cut from flat stock.
Below: None of this is secured.
From here I will move towards painting the stack, turret housing, and the guns.
Steve