Once the main hull is complete, and armored propa, cut a 3" piece of 1" PVC pipe to use as the main kannon.
You could use the FW big gunz or a piece of brass tubing to taste.
I scaled down this gun, but you can make it any size just as long as its dead 'ard and shooty.
Trace around your big gun and cut a hole to mount it in the hull.
I added an extra piece of 1/8" thick cardboard to the front hull and added a vision slit.
I got this card board from the back of an old writting tablet.
You'll have to cut through this too before you can white glue and mount yer kannon.
This pic shows the kannon and front plate mounted.
The vision slit was cut into the front plate using a dremel moto tool cutting wheel.
This thicker card is a beast to cut with a xacto knife so I used my Dremel Moto-tool to carve it up.
This method also leaves a better finish on the edges of your card board plate.
You could substitute platic card for this piece and get better results, but it's more expensive than free card board.