If I want to create my own designs, I would require design software and a steep learning curve into used that type of software.
Obviously I don't want to spend a lot of money on something I don't yet know I can and will work with. So first online searches are for free available 3d designing software. Off course that software will be limited in some way when making designs, but for a first start on designing some simple things, I should be ok. Once I'm more into making 3d designs and the designs and solutions become more complex, an investment into 3d software will be made to ease the work and have some more soffisticated features available.
I don't have any real requirements at this point, and there is a wide variaty of 3d drawing software available for free online. So in the end I came down to Design Spark Mechanical Explorer. Don't ask me why, because there was no apparent reason for that.
After download and installation I ran it and, as I'm always eager to just try stuff, started clicking and drawing without any look into manuals or tuttorials. Well... usually I can find my way around quickly in software programms, but although I was able to draw some basic stuff, I couldn't really do what I wanted. So I went into some instruction tutorial and spent half an hour on learning some simple tasks.
A second time in the programm I was able to some basic first drawing for what I wanted for my first project, but as I forgot to take the exact measurements I didn't continue.
Next time I was able to make a simple box with holes into it for storing some adapters. (See Project 1 in the Projects section)
For now it works, but as mentioned, I do have a feeling that I might need to invest in 3d designing software at one point to have some more advanced features.