Hey madram,
Like you I have 4 Turbinas on my GTV and a standard steel wheel as a spare, though mine are stamped Cromodora rather than Campagnolo. The centre caps on my wheels are 55mm in diameter, all metal and are fixed by a rear cap and screw. I took one off and photographed it.
I've seen other cars with these wheels and a variety of centre caps, so I'm not sure what is original and what is not.
You could try other BBs as these wheels are presently not popular and I believe owners are getting rid of them in favour of the GTA styles. You may be able to pick up an extra wheel plus some better fitting caps.
Hope this helps,
Chris

The pictures are yours to use.
As to the wheels, they were on the car when I bought it many years ago - apparently a factory option according to the previous owner. I rather like the pattern, but as I said, at present the Alfisti seem to turn up their collective noses at them.
If you are a member of the AlfaBB (a US bulletin board at www.alfabb.com), do a search under "Cromodora center caps" and have a look at post #7. Some guy with the login Pat Garrett cast a set in pewter from moulds that he made. They sound very similar to my centre caps only mine are alloy. I've attached a photo lifted from the BB and if you contact him, he may still have some for sale.
I look forward to reading your article.
All the best,
Chris

PS: Here is the link http://www.alfabb.com/bb/forums/alfa-romeo-parts-sale-wanted/59453-wtb-cromodora-center-caps.html