MVB's are not really a great choice for small enclosures. The are also much more expensive. My preferance is to use standard household Halogen floodlights for basking the bulb. These are readily available and cheaper than the "reptile" bulbs at the petshops. They also put out a clean bright white light which dragons are attracted too and a lot more heat per watt. In your 20 gallon Long tank it would probably only take a 50 watt to produce the right basking temp. That saves you significant both on the cost of the bulb and in electricity used.
For the UV light I use and recommend 18" inch 10.0 0r 10% ReptiSun or ReptiGlo tube lights. I also recommend mounting them inside the enclosure using a cheap 18" under the cabinet mount fixture you can get at Walmart for about 10 dollars.(Note: These fixtures come with a clear plastic shield that covers the bulb. Make sure to remove and discard this shield as it will cut of all of the UV output.) This can be mounted to the back glass wall and side wall about an inch under the lip of the tank so you can still slide the lid closed and using a hot glue gun.
Here is a picture of what I am talking about. This is in a 40 gallon Breeder tank but they sell a slim-line fixture as well which may work better in your 20 gallon tank. You can order the slim-line version for about 14 bucks at Walmart.com. The type in the picture is sold in the stores.
Mounting the UV this way will give your dragon full unfiltered UV light. The screen top of your enclosure can filter out up to 50% of the UV of your bulb when the light is placed on top. With the light unfiltered you will only have to change the bulb out every 12 months too instead of every 6 months with an above the screen mount which saves a lot of money again as well as being better for your dragon.
It will also then allow you to line up the basking and UV lights easier like I had shown you in your other thread so that both shine down on the basking spot making them both highly efficient.
The foil in the pic around the bulb is just heavy duty aluminum foil and there to intensify and direct the UV at the basking spot.
Again these are inexpensive and quick fixes that can be done in just a few moments and when you compare prices you will see that this way is much cheaper than a single MVB as well too more efficient and safe for your dragon.
Enjoy!!
