I've been running sumpless for years, since day one. Someday...... perhaps..... I'll also have a sump, basement is the plan. In the meantime, my ATO is fairly simple with a minimum of electrics, lots of clutter around the tank though without sump, the industrial look is where it's at! 7 gallon glass winemaking carboy on a stand next to the display. An air pump on a timer pressurizes the carboy via tubing through rubber stopper. Rigid tube to bottom of carboy (also through stopper) feeds fresh water through a mechanical furnace humidifier float valve into the tank.
Some dated photos to illustrate, I shortened the mechanical float valve by one or two chambers to reduce the footprint in the display. This has worked for me with minimum hassles for many years. Relatively fail safe since the top off is a mere trickle & there's just sufficient air pressure in the carboy to keep the water moving on demand. If you keep the top off container smaller, there's even less chance of a flood. The 7 gallons keeps my 77 gallon tank topped off for 7 days.... lucky 7!
There's also a gravity feed variation of this scenario. If you can mount the top off container above the display, all you need is the mechanical valve & some tubing.
Carboy containing DI water & tubing. Sometimes I mix Kalk or Alk into the top off water.
Mechanical float valve. Not sure how you can work your hood around this, but I don't really see that you have another option other than drilling your tank.
