HELP! Overflow box detached

While battling a cyano outbreak, I did a 10 gallon water change on my 46 gallon acrylic bowfront that has a 20 gallon sump. I normally only do 5 gallon water changes and this was the lowest I had the water in the display since I had it (tank is only 8 months old). Without the pressure of the water in the display, the in tank overflow box I had installed fell off. I had attached it using silicone, laying a nice bead on both sides of the box. I realize now I should have tried to weld it on, but the box I purchased did not have a nice smooth and flat surface to weld..

Here is the overflow box that I have: CPR RSS Overflow Box Retrofit Surface Skimmer - 12. It is available on MarineDepot. I do not have enough posts to past the link, but it is the regular model, size 12.

Any suggestion on how to re install the box? I do not have a second tank to do a full tear down to weld it on. Would super glue (coral frag glue) work? Is there any silicone that will cure properly under water that is fish safe? Epoxy? I am pulling at straws here. I do not want to put my livestock in danger while trying to re install the box.

I am not sure what I am going to do, but right now if I have a power outage, I will probably flood my apartment...