Aye, tbh if your bag's big enough and heavy enough to be a problem here it's probably also big enough and heavy enough to snap you like a breadstick if you land on it in an accident
But we're getting off the question... You could buy some rubber foam from somewhere like Frosts or VWP and just stick it on top, it'd be a bit of a shame I think but it'd do the job. Or, something like ventureshield paint protection film would probably help too.