I wouldn’t handcuff Ekeler at all, to be honest, and I do own him in my main dynasty league. Ekeler’s value comes from his receiving volume. I don’t see any of his backups scooping that up, should Ekeler miss time. And the rushing volume alone isn’t enough to make a Chargers back fantasy relevant, due to the lack of goal line carries.
Handcuffs only make sense if the backup can step into more or less the same role, in case the starter goes down. 2020 examples were Mike Davis in Carolina, or Wayne Gallman with the Giants. But when Ekeler was injured last year, none of his backups provided any notable fantasy value.
If you absolutely want to back him up, I’d recommend Rountree, as he offers the most upside.
But I’d rather look at players like Tony Pollard, Latavius Murray, Nyheim Hines, A.J. Dillon, J.D. McKissic or Gio Bernard. Sure, none of them will benefit from Ekeler getting injured. But they may still offer more standalone value even with their RB1s being healthy, than any of the Chargers handcuffs without Ekeler on the field.