The main guns look impressive, but are quite simple to make. First, chop up the brass into the correct lengths. Note the two felt-tip ends cut to size. One has the simple 'U' mount for the 37mm gun already placed on it...
The 'leaning brace' is simply bent out of copper wire as on the plan, and soldered in.
Here's a late update on making the distinctive bow 37mm gun!
Take a length of mains copper wire, and hit it with a hammer. That makes a flat strip.
Then bend the strip round a felt-tip to make a semi-circle.
Finally, bend the ends of the strip in to give a 'horse-collar' shape.
Cut two slots in the breech of the gun barrel, and solder in.
Now cut out the shield and the mount from brass sheet, about 0.010"...
The gun mount is simply a 'u'-shaped strip of brass with a bolt in it.
The actual mounts were often made locally to one-off designs - here I have filed the mount to a suitable shape.
Fill the gap between the two 'horse-collars' with thin card or brass sheet
And finally put a couple of handles on the end of the gun - these were often one-off designs as well...
The mount is soldered to the shield in this position...
The 37mm gun is simply a brass tube soldered onto a 'U' section. Here I used a square section with a bit cut away...
... and then the barrels are cliped and soldered onto the mount. Simples!
The 20mm Oerlikon barrels are three tubes - just get the lengths right and then solder...