If you are constructing your sukkah and then putting it into its final position, you are not using what is already made, you are merely finishing the construction project. The placement of the sukkah with the schach on it is still in the construction phase, no different from assembling walls and then lifting upright into position before nailing them to the floor.
If you truly couldn’t use what is already made, you would not be able to buy finished dimensional lumber such as 2x4s or sheets of plywood to build the sukkah, you’d have to cut down trees and hew your own boards