what did you pick in the end?
I have had good luck with the "Monoprice Universal Bookshelf Speaker Mount System"
I have not used them with the Ls50 so YMMV.
The specs say Bookshelf Speakers with 5.4" ~ 11.0" Width and 55 lbs max.
the Ls 50 does better much further out in the room but sometimes speaker location choices are limited by other factors,
I would not use the side holes in the adjustable width side brackets.
My Ls 50 meta's have four rubber feet and if you look in the hole in the center you will see it it threaded for an M8 bolt (see attached picture)
I think the baseplate on the monoprice bracket is not big enough (front to back) to support all four feet but you could drill holes near the front of the adjustable L shaped side brackets for the front 2 feet then use a couple of adhesive feet stuck to the bracket as far back as possible to support the rear speaker
This would give you 2 bolted connections, the side brackets for support and the adhesive feet under the speaker near the back.
Good luck, I hope this helps.