PLT OF PA31 ADVISES RECURRENT PROB WITH OVERWING BAGGAGE LOCKER DOORS NOT SEALING. ITEMS WITHIN TEND TO GET SUCKED OUT BY THE LOW PRESSURE AIR.

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A FEW SECONDS AFTER TKOF; PASSENGERS BEGAN TRYING TO GET MY ATTENTION. AS THERE WAS NO IMMEDIATE INDICATION OF A PROB; I CONTINUED CLBING TO A SAFE ALT. AT ABOUT 500 AGL; I BEGAN TO LOOK AROUND AND OBSERVED SOMETHING BLUE BEING EXTRUDED FROM THE L WING-LOCKER DOOR CRACK. I ADVISED THE TWR THAT I WAS RETURNING TO THE ARPT AND WAS CLRED TO LAND. AFTER SHUTDOWN; I FOUND THAT A BLUE PLASTIC SHOPPING BAG WAS PARTIALLY EXTRUDED FROM THE DOOR CRACK. THE BAG WAS FULL OF UNKNOWN CONTENTS; WHICH PREVENTED IT FROM BEING COMPLETELY SUCKED OUT BY THE LOW PRESSURE IN THIS AREA DURING FLT. I REPOSITIONED THE BAG IN THE WING-LOCKER AWAY FROM THE DOOR CRACK AND CONTINUED THE FLT. THIS IS THE SECOND TIME I HAVE HAD A SIMILAR OCCURRENCE IN PA31 ACFT. OVER TIME; THE WING-LOCKER DOORS DEVELOP A CONVEX SET TO THEM WHICH PREVENTS THEM FROM SEALING COMPLETELY. THERE IS NO DANGER FROM THIS TIME OF OCCURRENCE; EXCEPT THE DISTRACTION TO THE PLT FROM THE PASSENGERS DURING A CRITICAL PHASE OF FLT. THIS INCIDENT COULD BE PREVENTED BY A REDESIGN OF THE DOOR OR MORE CAREFUL LOADING PROCS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR ADVISED THAT HE HAS FLOWN AROUND 20 DIFFERENT EXAMPLES OF THIS ACFT AND THAT EACH OF THEM HAD SIMILAR PROBS TO A GREATER OR LESSER DEG. HE DOES NOT FEEL THE SECURITY OF THE DOOR ITSELF IS IN JEOPARDY; BUT HAS SEEN AS MUCH AS 1/2 INCH OF GAP BTWN THE NACELLE PROPER AND THE DOOR ITSELF.