First I want to apologize for my bad english.
Thats why I'll try to explain with more pictures (1 picture is better than 1000 words) :-)
First you will need a sketch with the dimentions. I used Corel Draw to scale the aircraft. I printed that part - with the antenas and used it as template.
The sprue from the kit is with trapezium section as shown on the pictures bellow:
I glued together 3 pieces. Because of the shape you can combine different positions to obtain roughly the section you needed.
Cut the necessary lenght. I used a file, the template from the sketch, a little bit