Downloading someones designs and printing a real thing from the internet is great and all, but nothing beats designing your very own items that address exactly your use-case, however niche.
There’s a lot of applications out there for 3D design, ranging from the powerful and complicated to the quick-and-easy. For anyone getting started though, I would recommend jumping in with TinkerCad.
TinkerCad is a really easy to use software that runs in the browser, so it doesn’t require you to install anything and you can be up and rung in a matter of minutes. It was originally designed as a tool for school kids to get involved in 3D modelling, but it remains really popular among the n00bs and the more experienced 3D printers alike. Once you create a new project on Tinkercad, you’ll be presented with the canvas where you can bring in all manor of shapes from the menu on the right. You can drag a ruler onto the canvas and it’ll show you the sizes of any of you model elements, where you can adjust them in increments as small as 0.1mm. You can combine multiple shapes into more complex models and merge them.
If you feel like you’re struggling to get the kind of detail you want from Tinkercad and you require something more powerful, the natural step would be to Fusion360. It’s free for non-commercial use so you can download it right now. The learning curve here is quite a bit more steep than tinkercad, but there are plenty of really good tutorials online. If you want to download it, click here.