Hull Specific Components

[Makinus]

I wish to mod a component that can only be used in a specific hull size, how i do it? Using mounts? How? There is any way to do it without using mounts?

What i wish to add are 2 components named "Commercial Charter" and "Civilian Charter", they would be 0 resources and 0 tonnage components that would reduce maintenance costs of ships, but the first could only be used in ships 2 sizes smaller than current ship sizes and the second only in ships with 4 sizes smaller than current tech. The Commercial Charter would reduce maintenance in half, and the Civilian Charter to a fourth of normal, and obviously you can have only one of the component in each ship...

Any ideas in how i can mos easily to this?

thanks