The Acid Trap requires 3 part before you can craft it. Two of these parts can be found in The Temple, in a corner below a stairway, both on the opposite side of one another. The third part will spawn in the Challenge Podium in spawn after you claim your 3rd banner challenge reward. To start these challenges, knife one of the 4 ropes in the arena and the challenge will start.
There are 4 locations where you can build the Acid Trap, one near/in each God Tower. However, you can only build one Acid Trap per match, so you'll have to do it in 4 different games with it in a different location every time.
- For Ra and Danu, the trap can be crafted in the "corridor" between the arena and the tower.
- For Zeus and Odin, the trap can be crafted on the stairs leading to the top floor.
In the following video I show the part locations and also where the Acid Trap can be crafted: