Extremely interesting! I faced the same problem a number of years ago, when I was commisioned to build a 1/35th scale diorama including a Horsa. I couldn't find anything in the way of drawings/plans (and I'm ex-Paras!), even via the Airborne Forces Museum, who have a nose and part fuselage section. The production team for the movie 'A Bridge too Far' had a similar problem at the time, and made the full-scale mock-ups by taking measurements from those sections of Horsas still extant, as a complete airframe was non-existant, and using the Italeri 1/72nd scale model (now available under the Airfix lable). I solved the problem then by doing the same, using the kit as a pattern and scaling-up a drawing; the fuselage was made from balsa frames and stringers, and covered in card, rather like the 'old fashioned' flying models.
I haven't viewed the other links, but I have looked at the profiles and plans posted by Wurger. This is the first time I have seen plans of any kind for a Horsa, so thank you Wojtek, they'll be useful for a future project I have in mind!