Originally Posted by patrickscott
... he disconnects his two 5 gallon propane tanks and stores them in his garage.
I presume that you are concerned that the tank valves may leak so you want to have a secondary cover for safety?
I did a search and couldn't find a metal cover/seal either. That doesn't mean that they don't exist, but I suspect that they may not based upon the valve design.
This is why I think that.
Your (newer style) tank already has a secondary safety check valve built in.
The standard travel trailer propane tank uses an Acme Low Pressure Fitting. That fitting inserts into the inside of the valve body at which time the tip presses against the safety check seal to open it and allow flow. You may have noticed at one time or other that if the plastic Acme knob isn't tight enough that you get no gas pressure. In that situation the nub hasn't been pressed in far enough to release that safety valve.
As to an external cap, the valve connection style is not really designed to seal to any external parts. (The plastic storage covers are to keep the valve clean.)
Being that the newer style Acme propane tank valve has that built in spring loaded safety check valve it makes any external cap seal sort of unnecessary. There is also a built in tank pressure relief to vent the tank if the pressure becomes too high.
There is likely better description out in the World Wide Web than I gave here.