There was a plan produced some years back by a Canadian modelmaker called Greg Sharpe for a complete WTC with gas ballast system. AFAIK, it's the only time anyone has actually sat down and drafted a set of plans for a dive module. You could purchase these plans via a website at one time, but I don't think that is the case any longer, as the website is no longer active.
These plans were also published in the Sub Committee Report, unfortunately I cannot remember the issue number- but it was several years ago. I'm not sure if that back issue is still available in hardcopy, it's certainly available in electronic form, but at this moment in time, you have to purchase a DVD with several years worth of back issues which comes with rather a large sticker price, although there is an invaluable stash of knowledge available in those tomes.