I designed this attachment to hold markers at the back of the 4tronix Robo:Bit MK2.

The Robo:Bit is a buggy that is controlled with the Micro:Bit. The MK2 has been superceded by newer models, but there are still a lot of them about, particuarly in schools. I don't have other Robo:Bits to test, but will likely remix this to suit other models down the track.

I printed a dozen of these to use with about 100 pupils across the course of a day. I printed some in PLA and others in PLA+. A couple of the PLA ones were snapped at the base of the pen holder, but all in all they survived a pretty brutal test!

I printed with .4 nozzle and .28 layers. Attach to the underside of the Robo:Bit with 2 x 3mm by 8mm bolts. 

The markers that fit this are Berol brand, which a lot of UK schools have and can be found online easily enough. The Berol Notewriter is shown in the pics as an example, but the best fitting and marking ones are the Berol Colour Broad. You can force a Sharpie in there, but it is a bit tight - I didn't use Sharpies, as they aren't suitable for 100 kids in a gym hall!