Here is the template.
The two side profiles and the template of the walkway.
I designed the gun platform for 15mm, to hold easily two gun bases 4x4cm.
If you print it right away from the pic given, I'm not sure if it'll give the desired dimensions.
So, you may copy it in a Word file and change the dimensions for the scale you wish.
Keeping the same profiles, you may create as many straight sections as you like.
If you wish to have a wider parapet or walkway, you may redesign the middle and rear profile sections.
The inclination of the front section wall is IMO quite good, as you may judge from the other pics.
For the middle section of the profile you may use a rigid yet soft material, as cork or foam board, so as to carve the gun ports easily.