This is a customizable holder for long, narrow items like pens, pencils, brushes, etc. It is customizable for a wide variety of dimensions.

You can choose one or more risers, and how many you need will depend on how large you choose to make it. I made a 3x3 with one riser in the center, but if you go larger, you might want to put one in each corner.

When printing, you will need supports for the bottom part of the riser, but nothing else. I used PLA with .2mm layers and it works fine. You can use a low amount of infill, 20% should be fine.

I added a Tolerance setting, so that you can control how snugly the risers fit into the base and top layer. The default gives a good fit on my Ender 3. If it is too snug, you can increase the Tolerance value. If it is too loose, you can decrease the value. NOTE: The tolerance applies only to the risers. If you need to reprint for a different fit, you can hide the top and bottom in the customizer and get a file that just contains the risers. Change the quantity to 1 to make sure you get the right fit. Once you get the fit, you can print as many as you need. Of course you can sand the edges if it's too tight and use a thick glue if it's too loose. :)

Edit: I modified the customizer slightly to allow more precise setting for tolerance.

Edit: I added a sample STL file with 3x3 grid and 50mm high risers.