Lazaah, I think it will work. I'm guessing that you need 400W HPS for CP. If you put glass on the top of enclosure, and leave it outside of the enclosure, you'll less likely to have heat problem.
It may be easier to get a grow tent (in the US, you can get your size tent for a reasonable cost: e.g.
Amazon.com: Virtual Sun VS4800-48 Indoor Grow Tent, 48-Inch x 48-Inch x 78-Inch: Patio, Lawn & Garden), and build a shelf inside. Then you can cut out the top part, and replace it with glass. The disadvantage is that you can't utilize natural light (unless you modify). But using sun light in an enclosed space could be tricky with regard to heat regulation.
Here is my current small-scale setup. Mine is smaller than what you want. We have cool climate in Alaska, but still I need to worry about the heat in the summer (T5 HO is inside the tent). The grow tent is 4'Wx2'Dx5'H, and the DIY plexiglass case is 3'Wx2'Dx2'H. Since the light is outside on the plexiglass case, the heat isn't an issue for that box. For the grow tent, I use hygrotherm to regulate the environment.
Amazon.com: Zoo Med HygroTherm Humidity and Temperature Controller: Pet Supplies
The exhaust fan on the top left kicks in when the temp is > 86F. This removes the humidity (ambient RH is 30% in summer and 10% in winter), so the humidify kicks in to maintain 65%. I also reverse day and night during the summer. When the exaust doesn't come on, this grow tent can maintain 65% day 80-90% night humidity passively (i.e. without humidifier). Below the bottom shelf, I put 4 2x1" tray, which are filled with water. Because of the enclosure, this passive humidity tray can raise the humidity from 10-20% to 65%.
With the grow tent set up (15hour/day), my annual electricity cost is around $330 (from Kill-A-Watt meter).
Our climate in interior Alaska is basically desert with real low humidity. With the controlled envrionments, growing orchids became super-easy. A lot of people think that fertilizer is important, but I think that most people haven't optimized for light, temp, and humidity (unless you have a greenhouse or live in perfect climate), so it's unlikely fertilizer is limiting the growth capacity of orchids.
Anyway, my point is that you should make sure you are considering how to deal with heat. Larger enclosure is easier to control this issue, but you still need to deal with it.