Sure you can keep the cage dry enough to make the cage last, but there may be a problem.
To keep it dry enough for the melamine to last, is most likely to dry for the ackies.
Which leads to the question, If your going to be building a cage, then why not build one that works for what your going to be putting in it. In other words, a cage really should be designed around whats being kept in it.
ALso I am not a fan of the One inch of dry sand in the cage and then give them a moisture box. If you think about it, why not just keep them in the moisture box and forget the cage part thats not usable.
Please think about this, the reason for sand or dirt is, for a monitor to burrow. To burrow means to dig a hole thats longer then the monitor. Hopefully much longer. If its not longer then the monitor, then its not a burrow. So the dry sand becomes of no purpose. You might as well use paper towels or egg crates or something equally not as useful.
About front opening cages for insect feeding monitors. No the best idea, Cattle troughs and glass tanks work great for a few really good reasons. One is, the crickets or whatever insects you feed cannot get out. The another is, the monitors cannot get out either, another is, the substrate does not get throw all over the place(out the door, in the tracks, etc) Oh yea, and they are cheap.
Remember, these are only suggestions, you can do whatever you like. Good luck building, FR