This is probably just me thinking aloud, but with an enclosure that size I'd be thinking more along the lines of a vertical heat gradient than a horizontal one. That way the air up top is warmer and the air on bottom is cooler, which would make the snake go up and down to thermoregulate, which promotes exercise. Maybe giving a deep enough layer of substrate (say a foot, maybe) would solve the issue of the floor temps not getting as warm as you'd like, and also allow the snake room to burrow and for you to add giant, snake-hardy plants like palm plants. With that size you could also have half of the floor space be a giant water area, and do something along the lines of a paludarium, and eliminate half the substrate with that.
Of course, this is all under the assumption that I'm loaded and can afford to replace 16 cubic feet of substrate every once and a while, lol. The dreams are big.