Personally I have drilled a hole in the top of each gearbox, cover with tape, but saying that I very rarely lube the gears, they don't really need it.
Back in the day Slop never used to lube the old white gears and he said they lasted just as long. Two schools of thought on the matter really.
Also, with our weather, you tend to take the car apart to clean it way before you have to worry about opening it up to lube it!
G