For fluorescent lamps, you want to stick to the low end of the Kelvin rating, so 2700K or 3000K is best, 3500K is still OK. The problem with fluorescent sources is that everyone uses different combinations of phosphor coatings in order to achieve the color temp, so one 3000K lamp might actually have a different spectral output than another 3000K lamp, and sometimes these can give different results. But it would be pretty rare to find one particular brand that just would not grow algae at all, especially if you stick to 2700-3000K as all of these have at least a fair amount of red in them. It's when you start to get into 5000K and above that you start to lose more of the red spectrum and get into the "lamps that won't work" arena.
If you wanted to stick with just one fixture on each side or each screen, then what I would do is get one of the dome-shaped reflectors from HD or Lowe or Menards that is as big as the screen - you can get 10"+ diameter reflectors like this. Get the biggest you can. Then get a bigger CFL lamp. These are generally more expensive (42W or larger) because they are not in high demand.
Alternatively, you could get 2 smaller dome reflectors (don't get the funnel-shaped ones, stick with bowl or dome shaped) and kind of point them toward each other a bit, if you get what I mean, and then use a couple more readily available CFLs, like 23W or 26W.