Something the size of the MLC would probably not be considered a personal bag unless it was half full. the question is whether you'd be challenged on it by the GAs while boarding is another matter.

i see a fair number of knapsacks/totes that technically exceed those personal bag dimensions getting carried board as a personal bag without any issues however. i think if the GAs think it can fit under the seat, then they're OK with it.

just remember that they can always fall back to the stated regulations on the dimensions whenever they want to.