The benefit of egg crate is to keep expensive fish from going carpet surfing and ending up as french fries on your floor.
Moisture... unfortunately... will happen.
When I mentioned making the top of the canopy with some egg crate - it's for the ventilation. If you can keep it well ventilated - because heat rises - then there won't be as much moisture build up. If the top of the canopy is enclosed, the heat has nowhere to rise but out the back of the canopy, close to the wall - which will heat up your water more than you wish.
Also, I hope you are aware that you should either paint the inside of the canopy a light color such as white - with a high gloss primer and paint to keep moisture from seeping into the wood. OR, you can stain the inside of the canopy and apply a good coat of polyurethane on it. There yah go - two different methods to prevent water damage.