This is a costume I made for my 10-year old son. It would need to be resized for an adult or teenager.
The following are 3D printed (all my designs, except the helmet, which was designed by Jason McLachlan):
Shoulders, upper arm, forearm, hand guards, belt hardware, backpack: https://www.thingiverse.com/thing:2609041
I made the chest, abdomen, and shin guards out of floor pad foam. I'd post a pattern if I'd used one, but really it was a case of holding foam to his torso and cutting/shaping as needed. For reference, though I recommend downloading the free Pepakura viewer and starting with any of several free shoretrooper paper models.
The entire set got a coat of rattlecan sandable primer, then matte beige paint, then weathering with black and burnt sienna acrylic paint, then a coat of matte clear enamel spray.