We will start with the nose cone. It is a separate moulding so it makes adding the four 20 mm cannon a snap.

Start by using your dremel and rout out the index marks where the guns will go. Cut four (4) 1.75" pieces of 1/4" Inside diameter aluminum tubing. chamfer one end and use the tube to cut a circle of 1/16" balsa and CA a circle in the end of each tube.

Sand the inside of the nose cone around the gun holes.

Fit the tubes in the routed out holes in the nose cone, leaving .375" inside the cone and making sure they are aligned with each other, and when satisfied put a drop of thick CA on the inside. Be careful not to get any glue on the outside.

Kinda like this: