Unfortunatley this has always been a human factors issue and will continue to remain. This very issue of loosing cowlings in flight on the AS350 Gearbox cowlings and the AS355 engine and gearbox cowlings has been addressed in the UK - Please see
http://www.aerospacedesign.co.uk/ad_compliance.htm
For some back ground info.
PS had never had the chance to see the G-BOOV stbd engine cowling incident on film thanks for the link.