Kevin,
In response to your first question, I would create the cage larger than the 4x4x8 dimensions, assuming you are planning on housing the trio together. That said, I would not house the trio together at all. Females will fight with females, and males and females will occasionally squabble, especially over food items. Housing a pair in the same cage will prevent enough challenges concerning monitors in the Argus complex. I frequently have to separate Argus x Flavi pairs. A 4x4x8 enclosure would be the minimum enclosure recommended for an Argus pair.
As far as substrate choices, I would use dirt and lots of it. You can get this from your backyard, a nursery, or a home improvement store. You need the depth of the substrate to be at least 24". You can accomplish this by purchasing a stock tank, filling it with dirt and building the rest of your enclosure on top of and around this tank.
I would make the cage opening from the top rather than opening from the front. Try negotiating an adult male Argus out of a frontal opening cage, you'll soon see why. You can keep a front opening for cleaning purposes, but if it opens from the top, you can drop your food in and get to a monitor easily than coming at it straight on.
To line your cage, you can purchase FRP at Lowe's and Home Depot for about $25-30 for a 4x8 sheet.
I'm glad they are eating like champs, put some weight on that male. How large are the monitors presently? If they are sub-adults, they need to be out of that cage. These are very active reptiles, using every bit of their enclosure. I housed a hatchling in a fifty-five gallon for a very short period, within months he was ready for a much larger enclosure. Although you have them in temporary enclosures, the female needs to get out of that dog kennel. How can you properly respond to her needs or how can she have access to heat, humidity, etc. She is losing a lot of heat this way. I don't mean to give you a hard time, but you need to get them out of those temporary enclosures ASAP. You could construct makeshift enclosures in an afternoon using plywood and 2x2's.
The ArgusxFlavi crosses are kept and behave much like pure Argus. They can be slightly smaller and more easy to work with, but neither of these are very compliant to handling. Every single Argus or cross I have owned has hissed at me simply opening the cage, unless I have food, which they are eager to lunge at.
Here are some websites with some very helpful information:
http://www.pizards.com/hbd/argus.html
http://www.kingsnake.com/monitorfaq/species.htm#7.2
Where did you get these monitors? I sold a male/female Argus/Flavi pair and a young Argus male in December to someone. The size of your monitors is very similar to the size of the ones I sold. The young male was in great shape. It's easy to start jumping to conclusions. Would you mind sending some pictures. I wonder if they are the same three. I could send you some pictures to see if this is the case.
