Those cooling holes need to be with same dimensions in flexcaps so in 15" they will make already quite big holes to the capsel. If those holes are smaller plastic will go to white because it streches too much. Also in car tires the U-groove size is already guite small. It is difficult to make them hold so well, because the locking dont work with small groove.
I think also at this point there is no sense to drill any holes. If you drill holes you should different number of holes depending on the number of the holes or spokes on wheels. I might have tomorrow one clean solution for you if I receive the brackets I have ordered. I will put more data on that when I have it.
Does it really cost 15$ to send 18"x18"x1" package coast to coast?. Weight is about 8 pounds. I looked the price from united states postal service. Is there any cheaper place?
These will be manufactured in the US soil to avoid those high freight charges from Finland to US. Price for 4 will be 60 US dollars + shipping about 15$.
Send me private message if you want to order. I have planned to make the order on Monday evening on Finnish time. That is monday morning there.
Yes I manufacture these flat or even convex hubcaps at the moment. Price for 4 pcs of black abs covers and ziptie plugs for attachement is 123€ + shipping.