The term you are looking for is “parametric”. You might be able to find a design you need through the many 3D printing file sites using that term. FreeCAD and Fusion360 are open used but another is OpenSCAD if you are more programming savvy than modeling savvy. There are plenty of recipes for threads in there to make your relatively simple design request here.
Tons of good and free options, you just gotta pick which one looks easiest for you.
Tampermonkey will do what you need. It’s not point and click, but writing the custom script shouldn’t be too difficult for someone very new. You essentially just need to complete a find and replace upon page load. The %2F and similar costs are just ASCII characters in percent-encoding and can easily be parsed.
I’m unsure what benefit you get from this though. It seems skimfeed is just trying to capture which lists and posts get visited so they can adjust the site according to popularity. Doing this via an exit link removes any need for cookies (which I didn’t check for, so maybe this ain’t what they’re using it for).