Those fresh air habitats, if they are the ones I am thinking of, would be totally unsuitable for a monitor. Screen in general is bad because it does not hold humidity at all. Being that monitors require such a substantial amount of heat to thrive, enclosures with scree tops become ovens, with all of the moisture rising out of the top with the heat. Although savanah monitors come from relatively arid habitats, they do need a decent amount of humidity to survive. In nature, they get this humidity from their burrows, which is why a deep, earthy substrate comes in handy.
What I would recommend would be buying a galvanized steel cattle trough. Fill the bottom with AT LEAST 6" of soil/sand/decomposed granite mixture. You will have to experiment a little with this until you find a mixture that will hold a good burrow. Then build a top with wood and plexi with some heat lights built in to the top. It would look something like this (taken from the ProExotics website)...
I'd get 45-50 watt halogen bulbs (not the ones with the clear hexagon in the center as they create super hot spots that will burn the hell out of your monitor), and then build up the basking surface until it's close enough to the lights to attain surface temps of 130-160*F. Keep in mind that that's surface temp, not ambient temp. With the lower wattage bulbs you will be able to get the appropriate surface temp without turning the enclosure in to a giant pressure cooker.
Here's a link to a ranch supply website that has cattle troughs. The 8x3x2 one would be perfect for him...
http://www.stockyardsupply.com/page11/index2.html
Here's another link to PE's caresheet for ionides monitors. They require very similar husbandry as savanahs, so you might find some of their enclosure notes helpful.
http://proexotics.com/care_ionides.html
Also, Kingsnake.com and varanus.net are two of my favorite websites for monitors. Both have pretty good forums where people often post pics of their enclosures, so if you look through those, you might come accross some more good ideas.
Good luck with everything.