Proceed until you reach Riochi's second room. Jump the gap to the southwestern part of the room to find a mask.
This one's found during Tennessee's first room. After circling around and climbing the pole, turn around to spot the mask on a ledge in the corner.
Once you've recovered every treasure in the game, the arcade machine in the final hideout will be repaired. Boot it up to play Maximum Chaos, which combines all the previous arcade games into one massive gauntlet. Play until the end of the Spark Runner section. Before entering the portal, roll north to collect the game's final mask.
Note: For more detail on collecting the treasures required of this mask, see our Sly Cooper: Thieves in Time treasure locations guide.