I'm getting ready to build my first custom enclosure. It is for one male Bearded Dragon. I want to make it 6x2x2. Is that good enough for an adult BD?
I have a fairly large male in a cage with a floor area of 48"x19". I'd consider that a minimum. He spends so much time out of his cage that I don't really worry about it. Nonetheless I do have plans for a larger cage.
I am going to make it out of Melamine.
Good choice if this is the first enclosure you've built.
Where is the best place to put the vents? On the sides or on the bottom back?
Not a huge issues either way. Whichever is easier and/or appeals to your eye.
Also, how should I do the lighting in the cage?
The fluorescent tubes are fine being exposed in the cage. The CHE and the incandescent bulb can be covered with a screen guard of some sort.
Either buy a pre-fabricated product or simply make a frame that can be covered with screen.
If you're taking the time to build a cage I think you'd be happer if the lights were somewhat hidden.
Here is a picture of my bearded dragon cage. It's about 49.5" x 20" x 28", LxWxH.

Sorry for the lousy picture, the cage looks much better in person.
I like the extra height as it allows me near 6" upper and lower lips. This allows me to hide the lights and heaters as well as provide a deep substrate dam on the bottom. If the cage were only 24" tall then the viewing area would only be like 10" which does not look as good, IMO.
The cage uses a false ceiling design. I don't have any pictures of that specifically but can show you on another cage I built.
This one is much larger, here's a picture before the cage decorations went in.

The cage is 49.5" x 30" x 48". If you look at the picture and pay attention to the light coming out of the back you'll see it's a bit brighter on the top 8". This is because the upper 8" of the back of the cage is completely open.
In other words, the back panel of the cage is only 40" tall and stops 8" from the top of the cage. This leaves me an 8" x 48" opening all along the back-top of the cage to insert and access lights and heating elements.
From the bottom of the upper (front) lip to the top of the back panel there is a frame built from 1" pine stock. Here's a picture of that frame.

The frame is covered with screen.
The provides a lot of ventilation but also allows much of the heat to stay in the cage. With my bearded dragon cage I drape Reflectix insulation from the top of the false ceiling over the back of the cage. It's just loosely draped so the ventilation is still there but it reflects most of the heat back into the cage.
It might be more than you want to tackle for your first cage but it is a neat and tidy way to hide lights and heating elements safely from your animal(s). It also hides the ventilation and is remarkably efficient for having the ventilation at the top.
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Current snakes:
0.2 Gonyosoma oxycephala - (Silver/Yellow)
3.3 Gonyosoma oxycephala - (Green)
2.1 Gonyosoma janseni - (Black)
3.3 Gonyosoma janseni - (Black & Tan)