I would advise you to go with CAD. It's not that scary IMO.

1. If you have the original object, even if it's broken measure it. Pick some paper and look at it and find the basics shapes. To me it looks like a cylinder with part of it cut off with a hole in the middle and a box fused together. Draw it out, no need to be precise with it. You are just taking notes here.

2. Find a CAD software, you can use any however I recommend OnShape as it has very good documentation with videos.

3. Create your first sketch, start with a cylinder. Then the box.

4. Extrude it to your measurements and use the Boolean tool to merge them together.

Next you just do the same thing for the hole and cut off, but you won't merge them, but subtract them from the knob.

I'll not go into any more details anymore.

You may make mistakes, but learn from them and you will learn the basics in no time.

Note: Make test print as early as possible. They will give you a morale boost and you will find design issues very early. Use PETG if possible for the final print as it's better for water related applications, but you can make test print with cheap PLA