How can I add my own custom Environment Effect to the dropdown list in Properties or edit (not change texture map) an existing preset?

None of the 10 available presets are giving me the surface attributes I want. They either reflect the image map to much or not enough, darken, lighten, or otherwise alter the base color too much (satin tints the surface red... but what if I don't want red?), or don't respond to changing the image map (gold seems to ignore changes to image map).