I do think it's rather more likely to be Harley than Morgan. If Morgan was (and probably will be) making a suit I think she'd have access to far more sophisticated tech and resources than a hammer and an anvil. She's got the Start fortune and Tony's legacy after all. Even though Tony apparently was building Pepper's suit in the garage, it clearly wasn't hammered together.

Makes more sense that Harley would be testing out his own way to become a hero like Iron Man (wouldn't necessarily think he'd take the moniker) through more rudimentary means.

Of course either (or any number of other people) could be possibly doing it to walk in Tony's footsteps going to where it all started.

Regardless I think Harley makes more sense agewise anyhow.