Help me please!!! Cracked overflow?

[sask]

I was trying to drill out the grate on the overflow in my tank to get the water level down a little while still maintaining the same flow. Well, a piece of the overflow cracked out and I have had to make a piece of acrylic to cover it for now. My problem is, if the power fails now, my water level will go down to low and probably overflow the sump. Does anyone know of a product that I can seal in a new piece of acrylic with, that will safely(for fish, corals, inverts) cure underwater? Please help!!!