Lofty Mirror ??

I am planning to make a couple of bathroom mirrors in the style of the
"lofty mirror"--google has lots of pics. It is basically a simple mirror
with a wood frame on the left and right, but nothing on the top or bottom.
These will get daily use and I prefer not to have a traditional (4 side)
frame for mirrors that will get everything from shaving foam to toothpaste
sprayed on them.

In a normal frame, the mirror cannot fall out or down, so I generally put
some silicone on the back of the frame to keep the mirror in place. But
given the lack of a mechanical linkage, will silicone be strong enough to
keep the mirror from sliding down or out of a frame without a bottom rail?
Should I use other stuff, like mirror-mastic or the like, perhaps troweled
on the back?

Any thoughts on the actual frame design? I am planning to have the mirror
simply drop into a shallow rabbet (not a frame/panel groove) on either stile
(8/4 stock), with the thinner connecting rails (4/4 stock), get hidden
behind the mirror. But I have never done this before and am not sure
whether the left/right edges of the mirror need to be hidden from view.

Cheers, Shawn