Ship Modeller magazine(?) had plans of a McTaggart-Scott catapult in the late 1980s. I have the magazine and photocopy somewhere. I found them yesterday, but rather than breach copyright, I also did a search on the web.
There is a fantastic build by a Singaporean modeller over on Military Modelling and he has all the details of plans.
http://www.militarymodelling.com/forums/postings.asp?th=131085
HTH
Tony