Google sketchup is great for designing enclosures- the one above took about an hour. The thing to remember is each "side" of the enclosure also has a thickness. I've seen many people's enclosure designs, and they often forget that each wooded plank has a 3D shape as well, with a thickness, length and height.
I always design furniture before making it. It helps iron out the wonky bits
For example- the design above almost turned out a failure. The central beams in the picture, which form the central dividing slot, also perform a structural function. I am not fixing any of the glass panels, they will slide into slots in the wood. This means the front panel is barely attached to the sides. This is why I included two central dividing beams- without them the front panel would be very fragile, and easy to break. If I didn't design it first, I probably would not have seen this.
I always design furniture before making it. It helps iron out the wonky bits
For example- the design above almost turned out a failure. The central beams in the picture, which form the central dividing slot, also perform a structural function. I am not fixing any of the glass panels, they will slide into slots in the wood. This means the front panel is barely attached to the sides. This is why I included two central dividing beams- without them the front panel would be very fragile, and easy to break. If I didn't design it first, I probably would not have seen this.