You can buy hoods just by themselves. My 30 gallon came with no hood as it was second hand. I bought a basic metal hood for it from a big LFS that I occasioanlly visit.
Or if you don't want to do that the the DIY section is the best place to ask.
Go to that hardwear store. Buy a piece of plexiglass have it cut to size leave room behind for tubes and air flow. Put light on top and boom your done.