How do I connect the speaker firmly to the stand so that someone won't accidentally knock the speaker off the stand?
This in a way that is non destructive (drilling into speaker...).
Actually you want to decouple the speaker from the stand.
Balance, poise, grace, and a little luck !
They're not as wobbly as you would think.
Make the base wider and center the speakers.
My old A25XLs were perched on a stand for years with no mishaps.
I did move them closer to the walls for parties and holidays !
The pictured stands have threaded rod holding the top and bottom solidly to the schedule 40 PVC tubes.
They feel like one solid piece.
The pipes are sand filled, for sonic performance and weight. (like 30 lbs each....plus the speakers)
They don't wobble, but could be a crashing fall if stumbled on.
Same danger with tower speakers.
If you use sorbothane sheets or carpet anti-skid backer, the speakers kind of "stick" to the stands.
Also the laminate floor backer can work (like Pergo's padding)
Carpet backer
Pergo has two products SilentStep (better) and SoundBloc (thinner)