my idea for a secret lava hideout is based on minecarts. when you right click one, you zoom to it, through lava, without taking damadge. so, make a 1 block deep hole, as wide as you want the lava lake to be, go to one edge, and mark it with a block. in that spot, dig down 3 blocks, and make your room 2 blocks tall, with the bottom of the 3 block hole the floor level. next to directly under the hole you came through, place a block with a minecart track on it, and a strip of blocks with tracks on them untill you are out from under the place where the lava lake will go, you can delete a block above you, to give enough room to get off the minecart. then go back to directly under the hole you came through, and place 4 signs on the inside walls of the entrence, to keep the lava in check. In case you where wondering why you need to have the minecart and track a block above the floor, its so you can reach it from the surface. now its time for the exit.go to the edge of the big hole, and delete a block, making a hole down into the room. then place a block at the side of the big hole, just adjacent to the little exit hole you just made. now put a block on the side of that one so that it overhangs the exit hole. then put a minecart track on top of it, and a latter below it.put 4 signs, one on each side of the inside of the exit hole like last time. then, put a minecart on top of the track on the block outside the exit and delete the block, so the minecart falls onto the latter. finally, fill the lake with lava. there you have it!