These would just be 3mm or 1/8" thick round plates. If you want there holes for zipties its possibly but some might use different attaching method.
Other side smooth other maybe with little texture (etched).
Pricing will be about 40€/4 for abs and about 80€ for clear polykarbonate. Like written above
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15" are not a big problem. Tomorrow I will try to go to local wheel shop to take my own estimations which diameter works for best.
What comes to color like that light grey have to ask the pricing. White and black they have at stock and material price goes down with the order size. by order size it is calculated in hundreds of kilos.