NEM coupling specification

I've got the Trix OBB rolling road set (x8 wagons) but want to convert them to Fleischmann profi couplings. Now one simple solution would be to put the coach that carrys the lorry drivers with two different couplings (1 end with profi, the other end standard Trix to hook with the rolling road). However I'm thinking along the lines of making a custom attachment for the Trix wagon and would like to know where the NEM standard is published eg size of the rectangular box section/ height above track etc.

ps these Trix wagon have non standard connectors , thats why I need to fabricate something.