8 in series, 5 parallel rows, with a current regulator (or resistor) in each series set to limit current. (28.8V nominal source = 8*3.5Vf Luxeon ballpark (a little lower Vf in reality is typical)). If your source is VERY consistent (steady 28.8V source with little fluctuation), then you can design your series strings to be roughly equal in Vf across the string (mix-and-match LEDs) to get each string roughly equal (say 27V drop each), then resistor each series for direct drive to the 350mA current (not recommended, but simple and easy, and reasonably reliable if you run it a bit underdriven (say 200mA).
Or better but slightly pricier (around $2-3 if DIY from a resistor, mosfet, capacitor simple soldering setup; or linear regulator, caps and resistor) solution is a simple 350mA current regulator for each string. So long as you have at least 1/2 to 1 volt to spare from your source for an LM108x or DIY transistor MOSFET follower regulator (you almost certainly would), it's a simple solution. So you'd be out about $20 in parts for 8 DIY regulators, and then just wiring it all up.
Of course your source must be capable of 2.8 amps constant output, but most modern lead acid, NIMH, and LiIon of decent amp-hour capacity should be capable (alkalines really aren't though, they'll sag fast under that load).
