I use toggles all the time on my bracelets and have never lost one
yet nor had it come undone. However, that being said, what I do is to
add in a cross bar in the circle receiving the toggle bar. That way
the bar can be inserted and removed but it cannot “drop” out without
manipulation. I think the trick is to have the circle, square,
triangle, whatever shape you choose, to be big enough to allow you to
insert the bar end of the toggle clasp, but be sure that it cannot
slip back out without actually being manipulated. The photo is of a
bracelet I made some 15 years ago and I’ve worn it countless times,
and have never had a problem with it.
That being said, BEFORE I added in a cross bar, sometimes just
resting my arm on a table would be enough to allow the bar to slip
out of the ring. Since I added in the cross bar, I’ve never had one
slip out. You have to physically manipulate the bar into a vertical
position (at right angles almost to the circle) to get it out.