Before you get to much going on I'd just get myself a a patch of doubled strength glass cut and take some silicone and plug that hole you now have and get the proper overflow installed or if you drill that hole low enough ? and it may well still be fine then you can just buy the properly constructed overflow.. This is not a hard item to build But it has to be made the right way.. Its all that holds the water back should power or pump fail. I Look at if as if you are going to all the trouble to set a reef up best do it the way it should be done. And save your self Time money animals labor and aggravation later one.. I base this opinion on screwing it all up every way that can be and or having to physically fixed up what other beat me to in the screw up dept. Not trying to sound rude or crude or short or to rain on you parade and don't take it that away . But Hoss what I have seen many times is that some do it correctly and some just do it. (: You decide..How you want to getter done
Its your money and time..