I'm not having a go here, but I honestly think people over-analyse this
whole DIY CO2 thing. Anyone who has done home-brew knows how simple it
is, really. If it doesn't work the first time, try again! It hasn't
cost you anything more than pennies.

You don't need sealant. Drill the hole slightly too small, cut the
airline at an angle and use pliers to pull it through the hole. Mine
hasn't leaked in 6 months. If it does, big deal, just drill another
cap.

The height from the bottle to the aquarium isn't an issue; it's the
depth that the airline goes into the water that causes back-pressure.
I have two non-return valves AND an airstone AND the bottle 3 ft below
the aquarium, and the pressure is strong enough to produce a good
stream of bubbles. So use non-return valves, and airstone if you want.

Extras
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I use a small plastic juice bottle as a bubble-counter. The two non-
return valves are between it and the generator, and between it and the
aquarium. A non-return valve would have prevented the OP's flood.
(This small bottle is good for seeing if gas is actually getting out of
the generator bottle through the airline.)

I use a simple party balloon on a T-connection just after the
generator bottle before the non-return valve to act as a simple blow-
off valve. It's held onto the connection by a rubber band. If the
pressure gets too high, either the balloon will burst or the rubber
band will give way. It's never been needed, but the balloon has
perished once from the carbonic acid. (I'm more careful now not to
allow liquid into the airlines, so reducing carbonic acid.)

As another poster said, gelatin (Jello) is good for slowing the
reaction and making it last longer.

--
Nick