This adapter allows you to stack another MakerBot Filament Spool Holder by charlespax http://www.thingiverse.com/thing:7105

Base and tube height and diameter can be adjusted in the OpenSCAD script to get a good fit. I made the base height a little thicker than is necessary, but the extra space is there in case you have Spool Housings http://www.thingiverse.com/thing:8762