I honestly don't recall, but I believe when I installed my valves, I re-used one of the holes that was already in the firewall.
I may have drilled one hole for a large self tapping bolt.
I do remember that I used rubber washers behind the valve so it was as cushioned as possible.
I you choose to use that location, at least make a bracket that utilizes those holes for the solenoids and still properly mounts whatever valve you have/choose.
Rubber washers, I thought, was a good idea.
Zip ties are NOT fasteners. Their intent is for aiding in the bundling of "things" (usually wiring).
Don't half-a$$ it.
Bob.