It could be anything looking at it from that height up.
I would probably use Composite or Aluminium.
It looks flush to the cladding so as you say, screw through the face.
You will need cladding type screws. I could just say self tapping screws but self tapers generally have a tighter thread on them that are not good when used on the metal used for cladding as it tends to be thin and soft. so the screw just threads itself and doesn’t give a good fixing.
Easy solution would be tech screws, but they tend to have a hex type head on the screw and arent the neatest if screwing through the face.
Tech screws also come with a sort of flat head that requires a star bit, but ive never been a fan of those ones.
maybe the quickest to source and do this is 5mm drill through face and a black headed rivet. John Adam sign supplies sell these.
If you dont have a CNC router, you could just cut yourself the letters in waste or budget vinyl, stick them to the face of black composite sheet. jigsaw round the letter and file any rough areas. job done and your onto the next. they are up high too so i doubt any little imperfections will be noticed.
alternatively, contact
James at Group101
https://www.group101.co.uk
he will see you right and give you a quick turnaround.
remember you will require a cherrypicker for the job.
price this separate on your quote as they can be pricey. normally
£50 drop off, £50 uplift and £200 per day hire. still add a bit to that for your yourself.
then offer the customer to source the cherrypicker themselves if they want to save a bit. but spec the machine required. i tend to go with a Rough terrain cherry-picker with articulating boom. makes a big difference when fitting and handles whatever surfaces are below yous the best.