I have 2 X 2" bulkheads and I made the overflow out of 1/4" glass ... the size of the overflow and the size of the bulkhead will be determined by how much water you want to turnover ( ie size of the return pump ) ... by using 2 Bulkheads on mine it might be overkill but at least if I want to upgrade the pump later on I can handle the flow.
Will see if I can post some pics tomorrow ( fishies are asleep
)
Cheers