How long does your elegy need to be? That might help determine what you need to include. Keep in mind that an elegy is a poem; so if your elegy does not have standard meter or rhyme, at least try to give it a lyrical (songlike) quality. What you might want to do is divide your elegy into four-line stanzas. In the first, you could discuss Portia as a wife to Brutus; in the second, you could describe Portia as a Stoic, and in the last stanza, you could discuss who will miss her and why (especially since her husband does not seem to mourn her death much).
One other suggestion, I would write the elegy from Cassius's point of view. He is Portia's brother and demonstrates much more grief over the news of Portia's death than does her own husband.