I designed this extra tool and spool mount to fit on the top of a core one. The design takes into account the max size of the core one print bed so the parts are split accordingly. All four parts fit on a single print bed and take under 3.5 hours. 

Do with this what you want -- one side can be upgraded to the tool mount, the other to the spool mount. Or you can go for double one or the other, or just one of the options!

See the assembly instructions for which spare parts are needed. Also, the renderings are the best I've got right now -- I don't yet own a Core One, but if anyone is interested in posting a make, please do!