In general, I shy away from stainless, but in that application, they would be alright & probably more available.
If it's slotted, raised head M5 screws that you want, I might have some, but most likely in stainless. I can't promise, but let me know the length & I'll have a look.
Unless you seek the OEM or Concourse look, the last time I fitted a horn casting, I used the longest M5 button head stainless screws that would fit but ground a countersink on the underside of the head (90 degree) spinning them in an electric drill with an electric file doing the cutting. A file would do it but take longer. They look alright (to my old eye's!) & polish up like chrome. Or plated steel, if you like.....
