The experts recommend that you get one of those very protective foam envelope type carriers to put the lappy in, then zip it up.
Then, you put that assembly into a backpack speciffically designed to carry lappys. It is hard to do, as not all of the backpack's fit all the different sized lappys.
But check the net for the special lappy backpacks.
The last time I looked, I did not find a hard shell backpack large enough to carry my 17 inch Dell and the protective envelope.