If you knew all of the care that went into them, you wouldn't be asking what a lot of varanid owners deem a very basic husbandry necessity - a deep, moisture retaining substrate that the animal can burrow into. I have never seen aspen or other wood substrates listed as a viable option and they should not be considered as such.
As others have pointed out, the depth and type of substrate then dictates some of the design necessities of the enclosure. There are no cookie cutter out of the box enclosures you can use for this animal. You are going to have to custom build something, taking into account ALL of the husbandry requirements.
- It has to be large.
- It has to be able to hold a minimum of a 2' deep dirt/sand or DG/dirt mixture substrate which is heavy.
- It has to be able to have an area where the animal can bask at a temperature of 140 degrees.
- The enclosure will also need to be able to hold in humidity.
In other words, I'll be amazed if you can put together an enclosure filling those requirements in a week's time. Here's a hint, stock tanks make an excellent base for an enclosure. Topping it is going to be the hard part.
Carefully read the links that you have been given - starting with savannahmonitor.org, paying special attention to basking temps, ambient temps, substrate depth and type, enclosure size and diet.
All of the answers to your questions are in those links. It is imperative that whatever changes you need to make, you make them as soon as possible. If you can't afford them, or if they seem like too much work, find someone who can properly keep the animal.