If your Fit is like my Civic, the radiator is on one side (passenger side) and the the AC condenser on the other. I have a little more than half covered, with the AC condenser being completely covered (I rarely use it here) and I haven't had any problems. I would just watch the coolant temp with your Ultragauge, I have an alarm set for 210* and if it gets above 220* I can just blast the heater. But the temperature barely reaches 70F on a hot day here, so I don't have any problems. You'll never know till you try it yourself.
(I have a coroplast one now that is the same size, but its painted black and difficult to see.)