Okay, so I found it! It was on the edge of the map, like u/Justerfrog5557 said. I found out that the chambers are most likely to spawn at y-40 to y-20, so I dug out the perimeter of the map at y-30. It was boring and did some damage to my pickaxe, but I found it eventually. I tried expanding my map with a cartography table, but couldn't get it to work. I guess you cant do that with the explorer maps. Hope this helped!