Please, Sire! I beg of you! Do not draw and quarter me!!!
I thought about this for a while. A person could simply sculpt armor for Gladiator. Then it struck me to simply use another body. You need one big enough to cover the chest of your posable figure. Look around for something cheap.

You need the chest, but also need the piece to drop down partial covering the bowels in case you get sliced open. I glued and coated the seam between the two. I like to use body filler or even a small amount of resin for this. It will bond tight and is sandable.

A slice of leg gives a shoulder covering, a ëdonerí shoulder gives the shoulder plate. The shield comes from a tupperware plastic container. 

Sand, prime and paint the works dark gray. When dry, try drybrushing in black. This is a technique I use to age or wear items. Put a little thick paint on the brush, blot brush on a cloth and slightly brush on object usually picking up high points.
In this case I wanted armor to look exactly like the movie, so I hunted down pictures and printed them at scale. You can print them out on a black printer and detail them with silver paint and a fine tipped paintbrush or even a silver fine point marker will work. See 1/6 graphics for shield, chest, shoulder and forearm.

Cut them out with an exacto knife and use spray adhesive to adhere them onto armor. Seal the entire armor with a satin urethane. 
Next, use a Dremel or smalldrill to cut holes in armor to thread leather straps. (Actually, this can be done ahead at primer time.). Youíll need two buckles the hold one side together. The opposing side can be threaded and attached with leather and wrapped around figure since only one side is needed to hold in place.