Use outdoor floodlights experiment with wattages. My larger cage uses 3- 45 watt outdoor floods to create a basking spot at least snout to vent length that is 130 degrees (dont exceed 150). The cage should offer a temperature gradient that ranges from the 130 degree basking spot to about 72 degrees to offer the animal choices. Hot ambient temps should be about 86 degrees. The basking spot needs to be hotter than what you have.
The full spectrum light is useless for a monitor because they get vit D3 from animal fat in mice, peeps, etc(whole animal foods). When you feed insects which you should also they should be supplemented with a good vitamin like sticky tongue farms "mineral" or one of a few other good ones are Flukers reptavite.
Out door floods take very little wattage to produce great amounts of heat, last long, are inexpensive (home depot 4.97), are very durable, and project the heat better(get the flood bulbs not spot bulbs, they get too hot in too small of an area).
Those reptile heat bulbs dont make much heat they are made to produce light that shows the colors of the cage etc. better, they are not durable and are too high wattage for what they do.
Use a piece of plywood as a basking spot its cheap and easy and holds heat good.
Aquariums are not good for holding heat, and humidity escapes easily. If you use one temporary (that monitor will quickly outgrow it) put plexiglass in the top to prevent moisture loss. Use a substrate that holds moisture for the animal. Dirt works great, cyprus mulch seems like it has a good following. If the monitor is kept too cool it can develop several problems; obesity, fat deposits in the organs, hair and substrate impactions, they are very lazy, and dont live very long. If kept warm enough and too dry they suffer from dehydration, stuck sheds, and organ damage that can result from dehydration. Any questions if you would like you can email me Ill try to answer them. Good luck..