In uk.d-i-y, wrote:

I've used them quite a bit and, if I remember right, I used 3.5mm x
25mm screws for fixing to 18mm contiboard. I have some 4mm x 25mm as
well which are useful if the 3.5mm ones pull out.

Aye. Go wild, treat yourself to a box-of-200 of each probably-relevant
size. The contiboard will be 18mm, right? So probably-relevant lengths
are 20mm, 25mm, and 30mm: go on, go wild and get some 35mms too. The
joy of not sorting through your old-sweetie-tin selection of pre-used
screws to find 16 screws all of the same guage, length, and head is well
worth the, what, 8 quid for all 4 boxes. You're bound to find another
37quids' worth of stuff to buy from Screwfix/IsaacLord/Axminster to make
it up to their no-extra-delivery-charge pricepoint ;-)

Stefek