Hi, I made a DIY Hood with 4 X T5HO for my aquarium last year, so I thought I could share some experience. The links to my post are below for your reference.
I used acrylic sheets as I found the material to be lighter and easier to work with, on top of that, water and chemical proof. It does not degrade or rust. Maybe you can consider using acrylic instead, and use glue or rivets to join them. They can even be ordered precut to the sizes you want so you minimise DIY effort. There is a link on my post to an online art shop that delivers precut acrylic sheets.
PVC trunkings were used for my light hoods, which rested on top of the tank canopy and can be lifted off to access the inside of the tank. For your setup, since you need to keep moisture in, maybe you can use acrylic to make the whole canopy and include a flippable top using acrylic hinges.
For the lights, I used waterproof end caps as they are not protected with a shield from water splashes. For your setup, perhaps there would not be splashing of water but making the light fixture water proof would prevent moisture from getting in, especially since it is in high humidity conditions. You can purchase all the necessary accessories you need for the lighting from DeLightings:
http://www.delightings.com/
For my DIY, I included LEDS as night lights and fans for ventilation. I supposed you do not need fans for ventilation since you need to trap moisture in, but they could be used for cooling your lights, if you design the lights to be in a light cover without airflow to your tank.
I would be glad to discuss further and do post your design and construction process to share with us.
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