Using the map indicated in this thread from Gamepressure, it's in the 3rd location from the far RIGHT of the map (according to the icons given). That is, the one to the left of the indicator for map 5.17. If you land on the top level, there is a chest there, but not the right one. Jump down to the next level and there is a chest there, also not the right one. Jump down to the third level - that's the right chest.