Take the parts with you to the hardware store. Go to somewhere like Ace where there is actually someone who can help you (not one of the big box stores). They should be able to sell you exactly what you need.
Am honestly not sure but thinking those heavy duty plastic barbed and ribbed screw thread covers might do the trick and even perhaps coat the covers with a heavy duty glue before screwing it in. I wish I remembered what they are actually called (and even tried searching for a photo for you) but you've probably seen them and can get them at any hardware store. A sales person would most likely know what I am explaining. If you've never used one before be sure to ask for proper installation instructions.
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