I came here to suggest what Steve did. You can replace the blade with a wider one; it will possibly move slower if you make it heavier but you can reduce the total exposure by cutting a narrow slot in it that passes over the hole. If you can't prevent the shutter blade from returning you will get a kind of double exposure though; one swipe of the slot each way. If nothing else, the results would look very funky with fast-moving subjects like propellers!