The walls on this model are just 1.2mm thick. With a 0.4mm nozzle on the printer, I simply set the thickness to 1.2mm to get solid walls on the exterior and interior dividers of the holder. The rounded columns at the intersections of the dividers are as solid as the 15% infill calculations can allow. I used Fusion360 to model the holder, and I've generated a link to a page where you can download it: https://a360.co/3MH6oJR It is designed parametrically, so if you want to make adjustments to the various parameters, you are welcome to do so. (Please adhere to CC licensing I've assigned to this model.) Of course, you don't have to use the parameters at all if you don't want to! :-) If you do make some changes, please share your remixes with me and the rest of the community here. I'd love to see the adaptions others come up with, and ways you make this design even better!