I put a single 400 on a 36"x24" tank by cutting a luminarc in half and extending it using mirrored AL sheet. Made the reflector about 30x20 if i remember right. Worked great and I had no shadows.
I made my own reflectors for my old 300g (36x72) and used 3 of them the size above.
For simplicity sake, Id just grab any of the decent brand reflectors and add some LED supplements.