Help with Grid Based Build System

Hello!
I am designing an Rust Survival Gamemode( Testserver: 109.230.252.185 Password=“DEV” ) and i failed to create a good Build System. I would prefer to use the GM Stranded Model Pack or PHX Props.

It should be like Placing a Foundation block free and then attach Other to this but i don’t know how to make this.
I think totally wrong how to solve this might be its simple but i dont figure it out!