I used a PAR 38 bulb from Wingo (from this site). I changed plans in my display tank, so had nothing else to do with the bulb. Initially, it turned the chaeto very pale, but when I turned the hours down, it grows it well. It's like a 10,000K or so bulb in a Home Depot clip-on fixture.
I don't think it's worth building a DIY for a fuge, because you don't need super-powerful lighting and hardly anyone will see it anyway.