I haven't bought from tenax24.de, but I bought a pair of long screws like these from eBay for £26.00:
View attachment 259957
Unfortunately I don't think the seller has any left --
BMW Z3 Tonneau cover securing fittings
However, I found that the tenax upper parts (with the clasp) wouldn't hold anymore, so it was a waste of time and money. Having said, I've used the short tonneau many times only attached to the side studs/screws, and I can guarantee that neither the wind deflector not the short tonneau will be blowing in the wind…