Only a few items are needed.
Button Set (located at a fabrics shop)
Small tubing (I used 3/32 OD) for vent
Ply wood (I used 5/16 ply)
Small scrap of balsa
The dome of the button set is 3/4 inch wide. So I started out with a square of plywood a little over 3/4 square. Locate the center and drill a hole through allowing it to be mounted on a mandrel on a rotary tool. It helps to cut the corners off making it look like a stop sign. For the shaping block I took a small section of pine board with a section of sandpaper on top. With the Ply in the mandrel, turn the rotary tool on and use the shaping block to turn the ply into a circle. When you have obtained the size you want then sand it with fine paper to get a slick finish. My finished diameter was a little over 5/8. I have a tip for my rotary tool that looks like an arrowhead. While the ply is still turning I use the point end to make the notch in the cap. Running at a high speed slowly form the grove. This now forms the neck of the tank.
For the vent try to form the tubing to look like the real one. I made three before I got what I wanted.
For the Cap drill a hole in the center to allow the tubing to come through. Easy way of doing this is to turn the cap upside down. Take a small nail and lightly tap it in the center, then drill. Take a scrap of balsa and sand it to fit in the top of the dome, Now glue it in the dome with CA or epoxy. Now glue the tank neck to the balsa. When dry drill the hole in the ply and balsa to accept the vent tubing, and assemble the vent.
Paint to your liking. The cap can be glued on the plane permanent, or be made removable. It’s up to you. Have any question feel free to email me.