The problem is, the lake will have an uneven floor so each block column will have to be moved independently with its own frame motor. And you have to leave at least one line of walls in place to hold back the lava while the other wall deployers leapfrog up and over the edge barrier to the new positions. Any low-hanging Netherrack over the pool will have to be cleared to make space for the frame walls, and if the pool gets wider, you need a wider barrier... seriously, a turtle has got to be the best way to do this.