Trying to put in add-on items to checkout...

Hi, I have included the photo below to explain what I am looking to do.

I want customers to be able to add a protective case to any figure added to basket that is tagged in my products with the tag 4". This is because we offer various different figures but only have the protective cases for this one size. I am hoping this is simple or at least possible? Thanks in advance for any help!

I’m open to suggestions if there are alternative ways for something like this to work?

I would prefer to do it without apps that become an added cost.