I may be of some assistance. I am finally, due to a new influx of money, abe to start building my new cage for my nile. Here is what I have to offer you, take it for what its worth. I dont plan on having an enclosure quite as nice as RobertBushner's blue beasts, but it will be functional.
The obvious thing about a monitor enclosure is that you need to supply the most room possible. Therefor, you should not be looking at the minimum size, but the maximum size you can provide. But to address a minimum size idea, it seems to be gospel that (1xSTL) x (2xSTL) x height as the minimum. Height for a sav would not HAVE to be as high as a tree monitor, however, like I said before, the more you can provide, the better.
I would go with side, sliding door entry. You can buy windows from Home Depot for $100 - $150 that are 5ft x 2.5ft, and of course they have other sizes, and these would work for your door. Or you can use plate glass on tracks, with a glass door key lock that stores use on display cases. either way, sliding doors are the way to go, because they are space efficient. If your enclosure is atleast 4ft deep and say 8 ft wide, top entry would be entirely ineffective, because thats a big freakin door. It would be pretty heavy also.
Other tips, I would allow for at least 1 ft of substrate, but as before with dimentions, the more the merrier. I wouldnt use the crap you have in there now, I made that mistake with another lizard. It does not hold humidity well at all. Go with cyprus mulch or a good, formable dirt(dirt being the hands down fan favorite) and you will be in good shape. I would build a few stacks to provide different temperature hiding spots, while being space effective.
The wood you use is up to you, i would use at least 1/2 in plywood, for durrability. This will work, or you can do what RobertB. does, and actually build it like you were building a house, with 2x4 studs and all. He even insulates it. In this case you can use waferboard. On the inside, FRP seems to stand up great against monitor nails and himidity, but seems to be a bit of a b**** to work with, as far as fastening it to the enclosure. Other methods of sealing are also out there, such as outdoor varnish and fiberglass boatpatch epoxy. The epoxy got a smell that packs a punch, and I was told about the outdoor varnish from a friend, but he has yet to see its effectivness. You will have to descide which way you go, just be sure you have a healthy protective layer on there, to prevent warping and rotting.
thats all i can think about for now. If I am wrong or missed something, I will be lambasted, corrected, or added to by our fellow herpers. Either way, you will get the info.
hope i helped,
Truth
PS - be prepared to drop several hundred dollars.