Building a hex canopy.

I have a 20g tank with 25w florecent hood. I just ordered 13w PC that I wanted to put behind the strip I have. The more I research about it I should make my own canopy. I could fit another one of these pc units in front. The bulbs I have ordered are combo white/blue. If I build canopy can I gut my current 25wlamp and use it? Anybody have any tips on building a canopy for such a small tank? The tank has LR with mushrooms and other stuff blooming that I want to provide light for.