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Ultimaker 2 Spool Holder Plug
If you remove the rear-mounted spool-peg on the back of an Ultimaker 2 or later, it leaves three oddly-shaped holes. This plug should be a light press-fit from the back of the UM2 and fills the holes flush to the inside of the build chamber.
If it fits too tight or too loose, adjust the feed rate by a few percent.