Forgive me if I'm overdoing this but I find it helps to solve problems by backtracking...
I'm assuming your overflow box is like this: A box inside the tank, a tube going over the side of the tank to an outside box. Inside the outside box there's a hole in the bottom. In that hole is some pvc-like parts - either it's fused to the box or there's two pieces that screw together with the lip of each side bigger than the hole in the box and the rubber o-ring between the lip of the box-hole and the lip of one side of the pvc-like parts. Those pvc-like parts are a "bulkhead" (which you might already know).
Are you sure the leak is from the bulkhead and not
* A crack in the box or pipe
* Coming from the pipes connected to the bulkhead. If a threaded PVC pipe is screwed into the overflow box, did you use teflon thread sealant paste or tape? That could definitely be the problem.
In my experience with bulkheads where the hole is in the side of the tank and not the bottom of an acrylic overflow box here's what's been the cause of leaks for me:
* It's not tight enough (hand tight should be enough)
* The o-ring (or rubber seal) on the wrong side of the glass - I think you want it on the side where the water is so the water pressure is pushing it into the hole, not away.
* The o-ring is cracked or broken
* There's a deep scratch on anything that contacts the o-ring.
If the water is leaking around the o-ring and it's a tight fit I'd try putting a thin layer of teflon sealant on the o-ring.
That's all I can think of without seeing the thing.