On the left is the top of the can, the red surface is the interface taper for the stock cap, due to this being a screw on I avoided that to avoid galling. Also if you notice the slot needed to clear the this wall that would get smaller and would not be machinable.
There are two options for mating surfaces in green, if the inside green surface is used the cap would be as you see it, if the outside surface in green is used the center cone section can be shortened up a bit giving more of a chamber. The outside green surface would interface with the shoulder on the outside of the can which is covered up by either the RR cap or this Flat one.
The simplest way is the outside shoulder but I believe I could machine either one. Of course there will be slots for the stock wrench.
View attachment 8717403