About a year ago I “designed” (ha ha!) and 3D printed the Super Boring Sanding Block which has served me well for sanding miscellaneous wood in the shop, but I found a nice parametric sanding block for OpenSCAD so I thought I’d give it a try.
You can adjust the length, width, and height, and basically make it any size you want. I made mine to fit the sheets of sandpaper I normally buy. I’ve now got six of them with various grit for each one so I can just grab the right one and get to work. (I also added labels to each using the Niimbot.)
My one complaint (which someone else also had) is that you can’t easily figure out the size needed to fit a specific length of sandpaper. I don’t know if there is some OpenSCAD wizardry or maths that would do this for you, but my solution was to print a really thin version (repeatedly) and adjusting the numbers until it properly fit my sandpaper, then I made full width versions. Things like sanding blocks are a perfect match for 3D printing.