I made a heater suit with multiple layers of fleece and some kink of super expensive miracle fiber, although I can't remember the name right now. I have a vest of primaloft and I love the stuff, light ,warm as heck, super compressable.
Make sure you have a wind blocking layer. If it's for super cold conditions I'd incorporate a hood into the suit too. All arctic gear has a hood for good reason. A wire in the hood would be nice. Put baffles on the zippers and one around the inside of the neck to keep heat from rising out of the suit. Some pockets inside for organizing stuff. You need a heavy duty zipper that can be operated from outside as well as inside. Also a slit in the back to run your safety harness lanyard out of.