It sounds to me like the sound people hear is probably an /e/ in IPA (the international phoenetic alphabet.) It doesn't really exist as a pure vowel in standard American English, but it is the sound that "é" makes in French or Hungarian, or "ee" in German. It's not a diphthong like the "ay" in "play," although many English speakers hear it as that. If you can stop yourself from gliding into the "y" part of "play," it's
that vowel. (A diphthong is a combination of two vowels smashed together, to put it simply. Most of the vowels we were taught as "long vowels" in elementary school were actually diphthongs and quite different than what "long" means in describing the character of a vowel for linguists.) I know one particular Spanish speakers who gets crazy when she hears Americans turn Spanish "e"s into the diphthong "ay."
This is just my speculation given the description. Reading up on a description of the Hawaiian language and the vowel sounds, it seems correct, with "e" representing a sound ranging from our American English "eh" to what I describe above, but a pure vowel and not a diphthong in either case.
That said, when speaking English, I think it's best to approximate using our phonology, so "poh-keh" or "poh-kay" would be equally fine by me.