The Antec 300 was originally a budget-priced case. But this is no longer true of the current versions. In the UK certainly it is priced not much less less than the Corsair Carbide 300R. Rather than attempting to modify the Antec buying the 300R could be a better option. The 300R comes with one front 140mm and one rear 120mm fan as standard which seems to be more in line with your cooling preferences. It also has a superior hard drive mounting system, more in line with current practice than the dated arrangement in the Antec.
Thanks for the suggestion.
The Corsair Carbide 300R has the same sideways HDD cage orientation as the Antec 300 Two (and P280 also - though the P280 has a somewhat less obstructing cage design), and thus the same front intake fan airflow obstructions, but at least in the Corsair they perforated it.
The thing I don't like about the 300R and many new cases is the proliferation of ventilation holes and fan placements seemingly all over the place. I look at the large grate on the top panel and all I see is just a big place for dust to naturally settle down into. It could make for a good exhaust, but having the exhaust fans up there just means more noise. Not only that, multiple side panel vents are more common - even behind the CPU socket now. I would just duct tape over all these various side vents. And the more duct tape, the uglier it gets. At least I can use black duct tape to match the color of the case.