Also, dont forget that you cant drill tempered glass otherwise it will shatter.. and then you end up with a broken tank and a broken wallet lol. If your going to drill holes in the back for the sump, then your aquarium can only be acrylic.
Something else that you can do that pretty much acts as a sump, besides the HOB
overflow, is get a large filter, or two depending on how big your tank is, and then take out the bio-bags and fill the filter up with live rock. This will act as filtration and a sump all together. Also, it will add to the biological filtration that you will be getting from the live rock in your main display. This is the route im taking because i dont have money for a overflow and my tank is tempered glass so i cant drill it. :/