And in addition, any bag I got, I would also add some hard plates (1/8" plastic sheets etc) into it to try & prevent any knocks should you come off.
Foam is OK for general anti rattle type stuff, but IMO you need something harder if it is going to be hitting something. (sheet/foam/laptop/foam/sheet).
If the laptop is expensive, then it's worth spending that extra on a very good bag.
And don't forget the users helmet to protect that too