Now that the exhaust manifold is is attached to the dive helmet, there is work to be done on both ends of the manifold — the vent holes at the back, and a hole for the exhaust valve shaft at the front.
Exhaust Valve Shaft Hole
The exhaust valve need something to revolve around, so I’m putting a shaft through the manifold. This shaft (just a dowel) will connect the handle to the chin button inside the helmet. I’m using a compression spring to provide the action.
But a hole that accommodates this dowel and spring won’t provide any “spring-back”. So I need to fill in half of the hole, in order to give the spring something to compress against. I’m going to drill out the exhaust manifold, into the helmet. Then I’ll plug half of the drilled hole with a piece of polymer clay rolled in a cylinder shape. I’ll get a rough shape, then cook it, then sand it until in can be pressed into the manifold hole. After gluing the plug in place, I’ll drill a smaller, dowel-sized hole though the plug. The shaft will have to be sized just right, so that the spring is compressed slightly against the plug when glued to the handle. That way, the spring is always keeping pressure against the handle and the chin button. And when the chin button is pressed, it compresses the spring even more, allowing it to spring back when the button is released.
The plug and spring. 
Plug in place. Spring is actually on the faceplate hinge, not the wooden dowel. 
Plug glued-in. 
Plug sanded flush, and ready for a hole to be drill in the center (see mark)
Manifold Vent Holes
After more sanding, to refine the manifold shape, I marked holes over the round end of the manifold. I tried to start marking in a uniform pattern, but it gets tiring, so the dots just get sprayed all over. After marking the dots, I drilled them out; just deep enough not to penetrate the helmet.
It’s easy to break through adjacent holes when drilling. 
After throwing on a coat of grey paint, it starts to look like it belongs.
