Well, I suppose I could try...but, well, besides the high probability of it sucking, I'm not quite sure where to start. I've only seen a full-size Warhammer 40K helmet sold on eBay once, and it'd already ended, and that's the closest I've seen from anything that looks like the T-51B helmet.
The two next closest options are either real military gas masks and paintball masks. Unfortunately, military gas masks just cover the face in a thin rubber holding and the filters look nothing like the T-51B, and paintball masks look too "slick" and doesn't have that bulky feel of the T-51B. I'd look more like Master Chief wearing that.
Any recommendations or suggestions? Know of any possible masks/headsets from other sci-fi universes that could serve as a possible T-51B helmet?