I assembly lighting domes and parts for hospitals all over the world, yet I wouldn't want to give building a 500G tank a shot for my first ever build, I dont doubt you can make a box and paint, Its more the sheer pressure thats going to be pushing on the sides and the way I see it if you do make a little mistake or miss a 2mm area with the coat it will most likely pop right open with that pressure.
Also I dont think wood is going to be so costly, I would be trying to source down a cheap/2nd hand panel for the viewing area, I read another poly tank yesterday and he got his second hand for $450 so your going to be looking at 600+ new, you wouldn't want that to crack.
Anyways I really hope you can get this working.
The glass panel Is roughly $180. That's with glasscages. Like I stated before, my one uncle owns a hardware store and he can get me glass cheap.
-Andrew