Here is my design that is an elegant solution for a simmer device for the pressurized stove design. It is the top 3/8" of a pop can with about 1/4 of the inside top near the pull-tab opening cut away. This will need to fit inside the vertical fin from the bottom can. The top of the disc must be above the burner holes to prevent the fuel vapors from wanting to go out the outer ring instead of the slot formed on the top. Filing the top of a second can until the top disc is removable makes the inside disc. Then cut away 1/4 of this similar to the first piece. The top disc will fit into the first piece and can be rotated to the desired opening.

Note:   I no longer use this design.  I use a bottom of a can inverted with a hole in the center about the size of a nickle.  This covers the burner holes and only allows the center  of an open top stove to flicker like a candle.