I re-read this thread, And this is my thoughts on your material of choice. For alot of herps, plywood cages are a good choice. If you are going to keep your sav. (which does look like you keep him well) properly, example- soil,lighting,waterbowl for soaking. All that dirt you will be putting in there needs to be keep damp, or the burrows will collapse, not to mention alot of monitors will flip their water containers. Now hears my point, the material of choice has to be able to with stand constant moisture and dampness. The cement board will not keep the wood dry, any concrete product absorbs moisture. This moisture will wick through the cement board and affect the plywood. So, the plwood will have to be sealed well, adding more cost and time to the project. If you are still going to go with plywood, line the enclosure with frp or some similar material. This will cut down on weight as well as being more appropriate for your application. Just out of curiosity, why are you opposed to using a stock tank. I can tell you that after using alot of different cage materials over the years, for keeping monitors a stocktank is very hard to beat. For my Ackies (now traded) tank, I cut a viewing window out of the front, bought a piece of tempered glass and siliconed it in place, i used plexi for the lid. Keep us posted on this, i'll give you my thoughts on this. This cage you are building is big and going to cost a certain amount of money, and time to construct. Think things through because a mistake is going to be a bummer. Build it right the first time. Clint