Found some $4 pizza pans at WM that look like just the thing for the 16" wheels on my van. The options I'm considering for attachment are either some clips that would run from around the "spoke" part of the wheel to the edge of the pan fashioned out of bicycle spokes, or velcro on the inside of the pan and attached with spacers and zip ties through the same part of the wheel.
These cover the valves, so nice clean lines that way, but they need to be easily removable for tire maintenance. I think the bicycle spoke clips would be sturdier if I can figure out how to do that, but velcro would be a little easier for removal, but I'm worried about whether that system would keep the caps on the wheels at highway speed. I'm thinking that if it's not just right, the rotational forces would easily pull the pan out far enough for the breeze to catch it and send it flying.
I submit this to you modder masses for your ideas.