Wheel cover idea 1 - Medium density foam mat
 

Have looked at a lot of the wheel cover threads, there are lots of solutions, but each one is specialised to it's particular vehicle/wheel.
Here's another one,
I do have some safety concerns, as this is a modification, particularly when using metal or hard plastic, if one should come off at high speed.
So I thought about using that medium density foam mating, like yoga mats, picnic mats, gym mats etc. you can get the fairly firm stuff about 20mm thick, cut to a circle and then fitted just inside the rim, I'm thinking velcro or plastic tabs glued to outside of rim to hold it in. If it is cut just marginally larger, then it will stay in place easily, can handle impacts because it is flexable, you can paint it up and if it comes off at speed, will slow down quickly and even if it hits someone or something there will be minimal or no damage.
I will get a small piece and try it out tomorrow.
Comments??

Sounds like you like foam! :)

It will work, but not as well as a hard exterior cover. Also, I'd think that it would get pretty nasty after it gets wet a few times.

I don't think water is an issue, they also use this stuff around pools for safety reasons. I have used it to seal around the engine bay and radiator, lasts no problems, but does go hard and cracks where I have had it directly in contact with the engine itself, not sure if it is the heat or oil or both.
A thin harder skin of polycarb or something like that could be glued to it on the outside, thought it was an idea to consider.

The lack of rigidity will probably make it less effective than something more rigid.

That's what I use for my grille block. It's getting brittle and the outer surface is pitted, after three years in service.

I worry about the thickness catching extra wind on the wheels, and whether it's stiff enough to not flap out and catch the wind. But please, try it and report back! That's what we're all about here.