On the original costume the chest box is also vacuformed, as is typically done with fan made costumes. Since I only needed one and I was still very noobie in the vacuform arena I decided to just build it out of sheet plastic, it's such a simple shape anyway. I drew up the templates in Adobe Illustrator based on reference pictures online. I printed these on cardstock and assembled a paper mock up just to make sure it looked ok, then rubber cemented the templates to plastic sheet to cut out the real parts.
Here's it is with a simple printout of the decoration taped to the front to see how it looks. The round parts on the sides are some kind of 2" pipe cap that I found at Home Depot. These appear to be styrene so were easy to work with. If I were to do it again I'd use a PVC pipe for the rounded bottom, as is I used .02 styrene supported only at the sides, and it's thin enough that it's a little fragile. It cracked a bit when I used liquid cement to attach the bottom rectangle, but I was able to repair it. The tubing is also from Home Depot, a washing machine drain hose. Not as flexibile as I'd like but it was cheap and looks good, though I ended up finding more accurate hose from Wampa Wear after much scrounging.
After a few coats of gloss white spraypaint I applied details details using stick on vinyl (even the "brushed metal"), most of which was scraps I got free from the sign shop that sold me the styrene for vacuforming. I drew up templates in Illustrator based on references and cut out the different color details by hand. The lights are a cheap 2 for $3 LED strip I got on eBay. These are flexible so I just cut off the extra LEDs and folded it over to get the 5/6 pattern. (had to cover 1 LED in electrical tape as their wired in groups of 3).
The greeblie details were cobbled together from a variety of sources, mostly bits from Ace hardware, which has a great selection of fiddly metal bits. The two buttons on the front are a guitar strap button (left) and some metal knobby bits from the hardware store, and another similar washer based knobby bit on one side. The gear in the third pic was scrounged from a tape dispenser, recast in resin and detailed with a resin cast of a lamp switch piece, painted with Alclad Chrome.
The last of the greeblies were a bit of an improv, as there aren't very good reference pictures showing these bits. I based mine roughly on the Dan Laws AT-AT driver armor, which is widely regarded as the best fan made armor around. These details are scratchbuilt from plastic sheet, scrap box bits, and some finish nailer brads bent 90 degrees, sandwiched in plastic. The backing is textured metal tape I picked up for a lightsaber project some years ago. I'm still mising the orange/amber button on the top, having not found a suitable replacement in time for Halloween. If you know where to get one or have ideas how to make one please let me know! :)
During final assembly I attached the chest box to the chest armor with a couple screws and some heavy duty velcro (why both??). A 9V battery floats around in there to power the LEDs, and I have 90 minutes of breathable air thanks to the high tech filtration and oxygen generation system. Well, there's a 9V battery anyway. Time to move on to the vacuform experiments!