Go to a plastics supply house, buy a sheet of 1/8 or 1/4 plexi (whatever thickness the glass is) and have them cut it to the exact dimension of the end pane on the 10 gallon tank. using a razor blade, cut the silicon seal along the bottom of the top rim, removing the square plastic rim thing. using the razor blade, or a VERY thin knife cut down along the seam of the end pannel. gently wiggle and pull up, removing the glass pannel from the bottom rim.
center and drill your hole in the plexi pane, just as you would any plexy job, sharp bit, small teeth, slowly, etc. using a small flat head screw driver, clean the channel of extra silicon in the bottom rim. silicon the bottom edge of the plexi pane, and work it down into the channel. silicon the edges and slide it into place, squaring it up to the sides. allow to dry. clean the excess silicon with a razor blade, taking care not to scratch the plexi. clean the channel in top rim, silicon it back on.
Boom, 10 gallon tank, with hole in side. I have seen this done on tanks up to 29 gallons, beyond that, you need to be good at cementing, because the pressure on the joints will increase a lot, and might not hold once you break the origional integrity of the tank.
