What you can do is go buy a tube of plastic weld. Cut out a piece of cardboard the EXACT size of the hole in your projectors dust cap. Then push the cardboard into the hole so its sorta snug in there. If there are some open spots, it's ok. Just make sure the cardboard won't fall out. The use the plasticweld and cover the cardboard you put into the hole and spread the plasticweld to make a seal from one side of the hole, over the cardboard and to the other side of the hole, making sure to put the plastic weld about a 1/4 inch past the hole to make a strong bond to the dust cap.
Let it dry. You can then either cut out the cardboard piece or just leave it. Either way this will 100% reseal the hole.
You can get plastic weld at pepboys or autozone for under $10.
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