I would go with the 8 spoke design and something like this
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Aluminium wheel cover attached with Ziptie Plug.jpg - Windows Live
Those are prototype ziptie plugs which will be soon at the market. You can also mill your own if you have a lathe. You need to use releasable zipties and it will work like a dream. That disk is 3mm thick smoke polykarbonate and is shaped little bit convex with some DIY spirit
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Oh and that is 18" wheel.
http://www.youtube.com/user/TunekoLt.../1/_K4HSD6c0_I