If you can figure out a way to cover the top you will retain a lot more heat in the cage.
Here is a link to a cage I built last fall and have been very happy with.
http://web.ics.purdue.edu/~echarris/archcage/
I have both a mali and an Egyptian, so I made the height suitable for the Egyptian, this has led to me having to use more lights than I normally would, but the cage retains heat wonderfully. If I were to do it again there are 3 things I would change for the mali:
1 - I would not make the cage as tall.
2 - I would use wood tracks for the sliding glass in front, not plastic
3 - I would let the cage air for another month before I put an animal in it (for a total of 6-8 weeks)
There is a cage building forum that is incredibly helpful here on Kingsnake, they helped me work through and avoid a lot of mistakes while I was designing the cage, and saved me a lot of wasted time and money.
If you have any questions let me know!
Elizabeth
ps - In the picture the glass is not in its tracks, when installed properly there is no gap between the top of the sliding glass and the cage.

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1.0 Mali Uro Archimedes
0.0.1 Egyptian Uro Zuberi Mosca Khu (Mosca)
0.0 Fish
0.1 Sulcata Minnie
1.1 Iguanas Flik and Loki
0.1 Newfoundland Jasmine
0.1 Feline Winter
Indiana & Wisconsin