There are plenty of free options. For more simple stuff I use TinkerCAD - which is a free web based 3d modeling software. It can export directly as an STL so you can print it. I designed my ground glass cover and shorter monorail for storage with TinkerCAD. Fusion360 which is way more feature rich is also free for "Makers."
TinkerCAD is a great way to get started in 3D modeling as it is very easy to use, limited in function, and has a large amount of online tutorials. I went from TinkerCAD to Fusion360.
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