Creating a minimal custom product field?

So now I have tried for hours to create my own custom field, but I can’t get it to work.
I really don’t get much use out of this example in the docs, it’s is way to complex and too few explanations of what is going on for me to understand. I Can create a new custom filter-field just fine but I don’t understand how to strip the filter and skip the loading of values from the settings fields.

Do you have a basic, basic example? Like how would I create a “custom” field, that is just a simple input-field with some custom html surrounding it?

Litium version:6.2

Do you really need to create a custom filedtype for this?
Maybe @patric.forsgard has a better suggestion?

This is a very basic example but does not save any value:

Thank you Marten, could you also quickly go over the steps i need to take to turn that readonly into a read/write field?

I also want to know this and want I created more advanced things later.