If you are willing to do some 45 degree angle cutting… A french cleat is one of the strongest ways of doing this.
Basically you cut the top of your piece against the wall at 45 degrees and the piece that goes over it at 45 too. When pressure is applied down onto the shelf it actually transfers some it sideways into the wall and means it will hold even more weight. I know it is a little tricky cutting 45 degrees on the laser though :). I have used a bunch of french cleats over the years and they are awesome.
Enjoy!
David.