I agree with those who doubt you need to do this, but having just replaced my deck fill fitting, the following seems like a possible solution for a "cover:"

If you have seen a household PVC waste pipe installation, you have probably seen a "clean-out" fitting-- a large diameter threaded plug. The female part of the fitting is sometimes part of a Y-fitting, but I think you can get a simple end fitting, which would be better. The idea would be to cut off just the threaded part of the fitting (1 - 1.5") and then cast it (as previously suggested) into the center of a pad that you mount over the existing deck fitting. The inside diameter of the PVC fitting has to be larger than the outside diameter of the deck fitting. I believe you can get the PVC in 4"; my deck fill has an o.d. of 3.625", so this should work. You would then have a large "stopper" that you could remove and tighten with a large wrench.

Some caveats:

I don't know how well PVC stands up to UV exposure.
You might want to lubricate the threads on the PVC fittings.
Once you have the deck fill encased in the "cover," you may need a longer "key" or other tool to open the deck fill. I'm not sure what is available or workable for this.