For what it's worth, Claude (CougarSE) over on the Fox forums used the Cougar center cap from the 15" wheel (pictured in first post) on a set of Turbo Coupe snowflakes. He claims that they would fit 10-hole wheels as well.
I have a full set of Cougar 10-holes (from the '86) and a full set of '89-'91 Cougar XR-7 wheel. Both sets have all centercaps.
I'll do some test fitting next time I have access to them.
1996 Mercury Cougar XR7
4.6 - Dynomax-Equipped - Tranny Cooler - 3.27 T/L - 125k Miles