I'm not really all that savvy with the FGO universe outside of Babylon and the Round Table, but if we're looking at the Fate franchise as a whole, then Elio would cosplay the following people:

🌑NO NAME <Archer> (Note: This includes all the variations and available outfits in the game.)

🌑EMIYA <Archer Class Card> (Note: A mixture of Shirou's Archer Card Install and the corrupted hero. Although, Elio's body would be more defined in comparison to their original counterparts.)

🌑Senji Muramasa <Saber> (Note: Stage 1 for personal preference and Stage 2 for aesthetic.)

🌑Amakusa Shirou Tokisada <Ruler> (Note: Elio prefers his Human form over his Servant form.)

🌑EMIYA <Assassin> (Note: Stage 3 for facial visibility.)

🌑Ozymandias <Rider> (Note: Stage 2 exclusively since Elio won't perm his hair. Would probably wear golden eye contacts.)

🌑Charles-Henri Sanson <Assassin> (Note: Stage 1 for less complexity, more mobility.)

These servants don't require a lot of work to physically emulate on Elio's part since the latter's body type and other physical features easily match their own to certain extents; in fact, it's possible to pull off a "perfect cosplay". Typically, he would pay for everything to make said cosplay more "official" (i.e. hire professional makeup artists, buy the cosplays online and have them modified by professional tailors for proper fitting, set up a professional photo shoot, the works).