I hope I am not too late!
A trick I use to quickly remedy this issue before it happens: (Incedently, a similar method is also in the 2009 Annual of Reptiles USA- guess I am not the only one that uses it...)
I buy a sheet of that white plastic egg crate sold at DIY stores as light diffusers. I then cut this to fit the entire bottom of the set up. Next I take a section of 1/2 inch schedule 40 PVC pipe and cut a dozen or so "rings" off of it about 1/2 inch wide. I then cut a slot in one end of each ring, about 1/4 inch deep. Next I glue these rings, slotted side down, onto the interior bottom of the tank with an aquarium safe adhesive. These act as supports for the egg crate. Next I cut a section of the PVC pipe two inches shorter than the interior of the tank is tall. I slot one end of it also. This gets glued into the corner of the tank so that the slotted end is 1/4 inch above the floor.
Once the adhesive is cured, I then place the egg crate on the supports. You may have to cut the corner of it so it fits around the long pipe in the corner. Now I cover the entire egg crate with one layer of that plastic needlepoint mesh so that it fits right up against all sides of the tank, and all big gaps are covered with it. Install your substrate, plants and so on as you see fit.
When you are not using the drain tube, keep a piece of sponge packed in the end to prevent animals or their food from getting into it.
Now, anytime you get too much water added to the vivarium, all you have to do is slide a siphon down the PVC in the corner and drain it out! This is way less expensive than attempting to cut a hole in glass and installing a drain. Another advantage is that you can see the water level in the bottom of the tank, below the substrate. If the white plastic line created by the crate is unsightly to you, you can cover it over with a line of tape the color of your choosing. I use black acrylic paint for this purpose too. Just leave a spot on the side so you can see the water level down there. You will get much more mileage out of your soil type substrates this way.