You will need to provide a enclosed area that is dry all year around for your Sand Monitor, and make sure it has access to a sand pit that is 4 to 5 foot square and no-less than 14 inches deep, so it can dig a burrow to rest in at night time and feel secure, the sand pit should be in the dry area of the enclosure and on really hot summer days wet some areas of the sand pit to make it slightly damp, so the Sand Monitor can dig fresh burrows that wont cave in all the time.
l use to keep 3 female and 2 male adult Desert Sand Monitor's ( Varanus Gouldii Flavirufus ) in a outdoor enclosure here in Melbourne metro all year around, the enclosure size was 12 foot long by 8 foot wide by 3 foot high, where the sand pit was 6 foot long by 4 foot wide by 14 inches deep, and each hide box was 6 foot long by 1 foot wide and 14 inches deep. they also had 1 50watt radiant heat panel attached to the roof in 1 hide box only, and 2 x 160watt radiant heat panels buried in the middle on the sand pit 4 inches below the top layer on sand, that provided them warmth on the cooler days and nights outdoor.
Here is a few photos of their enclosure for you to look at and hope this gives you some idears on your new enclosure for your Sand Monitor best of luck.