I was concerned about dust and dirt getting into the seldom-used jack on my MP4 player- 3.5mm Mic jack. I was refilling an ink cartridge and the kit contained silicone plugs for sealing the refill hole which happen to work great for sealing out dust from a 3.5mm jack. They fit almost flush so no snagging problems. I found them on eBay Canada cheap- 5 for $1. Nifty.
These are silicone "Plugs for refilling Canon BCI, CLI & PGI cartridges" .