I just thought of an idea. Why not just take a suction cup, run a sturdy string through it, and then tie that string onto the driftwood. Make sure that you have the correct length in the string. Suction cup to the bottom glass perhaps.
I dunno, it sounds like a "smaller" method to secure driftwood to the bottom. You could also tie a magnet to the suction cup, and suction to the bottom. Tie another magnet into the driftwood and that way you could just stick it together through magnetism. It will also make it easier for you to adjust the position and/or length of the string by just adjusting the string on the second magnet which connects to the driftwood.
I dunno, just a thought....