Is this the right/best way to template this ability?

This creature cannot attack or block unless you also declare a non-spirit attacker or blocker

See Okk.

• This creature can’t attack unless a non-Spirit creature also attacks.
  This creature can’t block unless a non-Spirit creature also blocks.

It's slightly functional different with shared turns, so anoter option is:

• This creature can't attack or block unless you also attack or block with a non-Spirit creature.

The difference is that the first wording allows it to attack as your only creature if a teammate attacks with a non-Spirit creature in a shared combat.

While "Spirit" can be used as a noun, "non-Spirit" is used as an adjective and would be paired with a noun.

Technically you might be able to remove the "also", and ten it would be able to attack alone if it somehow stops being a Spirit, but I suggest having it since it makes the intent clear IMO.