Well, the T5 experts will tell you that each bulb needs an individual reflector to reap all the light ebenfits that T5's are known for. So, with that in mind, sticking a sheet of reflective material under the lights isn't going to make for the most efficient use of the bulbs. The Ice Cap SLR reflectors were the supposed to be the best reflectors to use for T5. Tek came out with the Tek2 reflector but I'm not sure if the Ice Cap is still the king or not. I think the two reflectors do things a little bit differently in terms of light spread. Anyway, the SLR's are about $20 each, the new TEK II's are about the same. Then there are some generics that are a few dollars less and probably not as good. Or, you could try and find some of the original TEK reflectors which are probably on sale because of the new ones coming out. The original TEK is not supposed to be as good as the SLR's either.
Or, you can just put a sheet of reflective material under the T5's and get what you get. My Nova extreme is configured like that and it was still pretty damned bright. But since you are building one, I would think that you may as well make it as effective as possible.