STL Model Prep for Printing

Job ID: 39776351

Budget: $30 – $250 USD

I need to tune an existing STL file .
the front columns i need to be able to remove and reconnect.
i need a back panel added and flush with the back columns.
i need the top cover to fit model.

please understand that i am a beginner and I don't know much about 3d printing. I can download a file and prints that's it.