While I agree with what Ghostofman said as well as the quoting of the FaD Beta in that assembling a hilt doesn't require a roll per se, I can see/understand the desire for some GMs to have it not be quite so simple, especially those GMs of an older age bracket who recall that building a lightsaber was a big thing for an aspiring Jedi Knight.
As for what sort of check, I'd say let the player have the option of either a Mechanics or Lightsaber check, with the difficulty determined by the base Rarity of the type of hilt they are constructing. So a regular basic lightsaber hilt (Rarity 5) would be an Average difficulty check, while a double-bladed lightsaber hilt (Rarity 6) would be at a Hard difficulty.
It's not something I'd ever require in my game, but it's a decent baseline until such point as FFG opts (if ever) to include more detailed rules on lightsaber construction.

