If you're just asking about the location of the apostrophe, then you could get away with "Eels' " for both singular and plural, since in US English putting an apostrophe on the end of a singular OR plural noun ending in "s" will make it possessive.

We're going to see all of the Sims' home movies tonight. ("Sims" being the name of a family).

I went to Mr. Sims' office today.