You might consider adding support for dungeon structures too as they are present in chunk files in a similar way as other structures. (finding minecraft:mob_spawner in NBTExplorer is what I do now).
Can't enter the seed for slime chunks mapping. Whatever I do the seed remains "Seed: Unknown".
Symbols typed in the Seed field are doubled. Copy/paste seems to enter the numbers once.
I play on my own private local (127.0.0.1) vanilla 1.13.2 multiplayer server.