The build depends entirely on your champion. So basically what you want to do is take a look at this list and search for the champion that you want to play. You will see the amount on AS% that's needed to reach 2.5Atk/S. Now since this is URF you can basically take that amound and divide it by 2. Keep in mind though that some champions have spells that increase their attack speed and If you want to go for a pure 2.5Atk/S build those spells might become useless.
Anyways: now a little example:
I want to play Graves so in normal mode I would need 250.7% AS and here I need 125.35%. As you can see It's impossible to use every single % of Attackspeed because of these numbers. Now If I just wanna go for the 2.5Atk/S with a more or less viable build I would need a Phantom dancer + 50(100)%, a Blade of the ruined king + 40(80)%, Berserkers + 20(40) and another random Attackspeed Item (I think only PD would be viable but also a poor choice).
Keep in mind though that this example also shows a champion that has an attack speed buff which will be on almost no cooldown in URF. This buff also adds 40(80)%. So basically I'd say a Viable build for every champ would be
BotrK -> Berserk -> PD -> IE -> BT/LW -> Random Def/Aggressive item
because it's absolutely not worth reaching 2.5Atk/S since some AS spells will be useless and you will spend far too much gold on overcapped AS. If you want to get to those 2.5% AS go for a PD as last Item because you will at least gain some crit from it.