This is a 5x1x3 Gridfinity bin designed to hold any tubular item up to 16mm in diameter and 208mm in length and still be stackable. I designed this with the Xaomi Wowstick and its tubular bit holders in mind as I couldn't find other designs this compact, but it will work with any sort of bulky designer pen or paint markers that exceed standard Sharpie dimensions. Accommodating a 16mm diameter item in 21mm space was no small task, and as such the channels overlap with the Gridfinity base a tiny bit causing some small holes in the bottom of the print, but rest assured as stackability is still maintained.
EDIT: For whatever reason, the model looks strangely smoothed-out in the preview, but if you download and open in your slicer software it looks just fine.