Picked up force and destiny the other day and can't seem to fing the rules for construcing a light saber. Is there any rules for it or just a narrative device?

It's narrative. Actually assembling one is about as difficult as putting together a flashlight; aside from the crystal, the components are all commonplace.

Upgrading the thing with attachments and mods does take effort as per the standard rules for those, although force-sensitives have an easier time improving their own sabers.

Check out the GM's Kit and Endless Vigil for more in-depth look at lightsabre construction.

Picked up force and destiny the other day and can't seem to fing the rules for construcing a light saber. Is there any rules for it or just a narrative device?

The core book treats lightsaber construction as a simple matter; acquire the parts and then spend a small bit of time putting them together, with the hilt parts being fairly trivial to acquire and the crystal generally being the hard part.

In the GM's Kit, we get a set of lightsaber construction rules that after you acquire the parts as above, your character spends about three days assembling the hilt and attuning the crystal. The end result of the Mechanics or Knowledge (Lore) check (PC's choice as to which they roll) is that if successful, you get to add one of the crystal's modifications to your lightsaber for free (which only applies the first time you use the crystal to build a lightsaber). Advantage and threat can be spent to alter the construction time or cost in materials, while a Triumph nets you an extra hard point and Despair costs you a hard point.

in Endless Vigil, FFG rolls out a construction method similar to that used for armor in Keeping the Peace and pretty much every other gizmo in Special Modifications. You select a type of hilt, pay half the credit cost of what the hilt would normally run, and then make a Mechanics (and only Mechanics for this method) to build the hilt, using advantage, threat, Triumph, and Despair results to choose from various qualities provided.

Personally, I prefer the GM Kit method of the three options, since the end results of building a lightsaber aren't quite as screwy as the EV method but isn't quite as trivial as the core rulebook option. Though in regards to all of them, obtaining the kyber crystal should be a major event in the campaign anyway, so even the core rulebook option isn't really trivial as much as it's something of an anti-climax with how simple it is (Average Negotiation or Streetwise check to get the parts for a basic hilt).

Here's a narrative showing how a character creates his curved hilt saber from his crystal. This one's a little complicated. http://diceforbrains.com/dice-for-brains-podcast-s1-special-episode-xens-lightsaber/