Let me get the sizing straight, IIRC: the Arc Reactor is 4.75" in diameter, not including the ring in Tony's chest it's set into, right?
One of the advantages of building a model using the computer is that even after I've put the files up for you guys, Pepakura Designer has a built-in scaler for you to use. Downey is 5'7", maybe 5'8", which gives us a scaling reference so that your own Reactor doesn't look tiny/ginormous when worn.
I would love to see the pep file completed this would be fun!
The model itself is pretty much done, I just need to modify it for actual building, you know, support struts and the like.
My current thought on the clear ring is to take the cardstock printout and construct it inside out, with the tabs and gluing on the outside, leaving the inside clean. A gloss coat seals it, and then you can squirt (or pour) "thermoform adhesives" into it to cast your ring. Hot glue. I've seen someone actually make Guyver armor with the stuff, it's crazy.
Easy to manipulate, cheap, straightforward, and while not optically clear I'm sure that'll only help, as it would diffuse the diodes. The rest would be constructed with styrene or cardstock, along with a couple of found parts (like a faucet filter screen and copper wiring) and some paint.
Suggestions welcome, if you can think of a better way to do it without breaking the $10 mark, please speak up!