When putting new carpeting in my Mark V, the carpet plus pad thickness was greater than the original Mutocell, felt, and carpet thickness. I found the two fasteners on the dipstick access panel did not reach sufficiently. At that time, though I could find manufacture and supply for the same type of fastener, I was unable to find one of the length needed.
My solution was to take two spare fasteners, cut them in half, and weld in the needed extra shaft length. A benefit of this approach was that the part of the fastener visible from car interior retained its old look unchanged. I used two spares since I was not entirely confident it would work well and wished to retain the originals to the car unchanged. However, it worked well.
The quarter turn fasteners here, sometimes called bi-fix, are different from Dzus fasteners I have used on racing applications.