Hi Greg! and Don Too!
Listen guys, there is a good way to make these things in both Paper and Plastic. The paper ones first. Greg; Take the scrap strips from the edges of the parts pages. Find a piece of 1/16 craft foam. Now gently apply water to the strips that you have cut into small squares. Take the rounded end of a paint brush that matches the size of what they are supposed to be when done. gently press this end into the wet paper. Carefully press till you have created, maybe somewhat thinner, but paper domes for sure!
Put a B.B. in each dome, Or very small Model railroad ball bearings. Let dry. Cut the edging off and mount on your tubular base.
The Plastic one is just as simple actually, Don. Take the right size Rod or Tube and cut it for heighth then make sure the top is angled to recieve the Domed upper part. You can create them with Bomb noses. Car spotlight bodies. Or take some plastic.080 squares. Laminate three together for very large ones. Use a drill bit to start and drill a hole dead center. Then carefully taper the edge of the hole till you get the shape you want. Remember some are not round but somewhat of a Narrow Bottomed oval
Kinda like the figure's mouth in the well known painting-"The Scream". Or they may be a complete balanced oval. Or round and yes, some even are like a rounded Square with large rounded corners. Carve the outside to match, mount on the tube then paint. Mount on the ship. Viola' two types of material, two types of Vents. Go for it guys!
One little noet for all. Now you know why I promote found Material. I have some recently made from the very domed small end of Syringe safety covers!