Your top looks nice. I think it will hold humidity fine with a UTH source. If ambient temps become a problem you could try adding to the top, or covering one of vents with it. I really like your inginuity with the top!
My suggestion would be to get the tank temps and humidity stable, and then your set. Argentina is right that it is a lot of space. Either add more things to fill up the space or use your idea about using part of the tank. I would also use hides that allow the snake to be hidden. Doesn't have to mean more money, you could use small boxes with a small whole cut out. The hide should be small enough that the snake feels like he is in a secure/small area.
On hides most recommend using identical hides on both ends. The reason being neither one should be more preferable and your snake will thermoregulate. While I would not suggest a new snake's home to deviate from this advice, I have six snakes and in each cage no two hides are the same and all my snakes move from hide to hide. So try identical hides first for your snakes sake. Once he is comfortable you might want to deviate from this, if it suites you more aesthetically. Only my BP has store bought hides (spoiled). The rest move back and forth between boxes. Also all my other snakes will burrow, so often they hide without using any of my man made hides.
