I liked the design by julian3d, but wanted to adapt it to use some 6.4x1.6mm magnets I had lying around. I did a quick-n-dirty remake in Fusion, and punched holes on the bottom for my magnets to press into place. Step file is included if you have any ideas for improvement. 

Works great with the standard 10mm targets used by the Einstar and similar 3D Scanners. I printed with 2 walls and 5% infill in some kind of PLA, doesn't really matter. Probably put some extra bottom layers so it doesn't collapse when pressing in the magnets.