This one's a lot more fun with water than with lava but it works with either!
As always, pop it in the AVWW\RuntimeData\Logic\RoomMaps\GeneralBossRoom folder.
(I've removed the version number from the uploaded file as I realised when ported into the main game all my maps keep the first version number they were implemented with, for the sake of overwriting - this should keep things tidy!)
Enjoy!