Hi Ollie:
The 'shorter' answer:
Use
Push-Pull Tool to pull (aka "Extrude") the 'Selected' front
Face of the vertical tail stabilizer forward to a point that the curve starts its ascent up and to the rear ...off of the top of the trailer body.
Use
Tape Measure Tool to draw
Construction Lines (aka "C-Lines" or "
Guide" lines) ...on a side Face to create
End Points.
< Guides are your friends; invite them into your project as often as needed ! >
Guides can then be Selected individually and Deleted, or they can all be Erased "
when the party's over" via:
Sketchup Menu > Edit > Delete Guides 
Place anchor points of
Arc Tool between those Guide lines to draw a base line; pull out on mid-base line to form a curve with Arc Tool.
After the Arc is drawn, Select with the
Pointer Tool, and push the Face fully across the vertical tail stabilizer to the back side Face extent, to a point such that the Face for the (no longer needed)
outside portion of the curve '
deletes itself'.
I should have a 3D model of this object to post as an example illustration shortly.
I can also post some additional ideas for how to further enhance this 3D object if you like.
BTW: If you have not seen it yet, there is an abundance of excellent photos and 3-view (Top / Side / End) line diagrams which could be useful in making a precise model of the Cobra glider trailer(s) on the manufacturer website:
https://www.cobratrailer.com/?page_id=1149&lang=en
https://www.cobratrailer.com/wp-content/uploads/DoSi-Anhaenger-1024x768-150x150.jpg
FYI: The 3-view images of
ex:
DoSi Class Cobra glider trailer can be imported as a
Texture, and mapped as a
Material onto selected Faces of a rectangular block 3D object "primitive" made with these dimensions:
10680 mm Long x
2180 mm High x
2050mm Wide
You may note that although the 3-view line diagrams are 'fairly' accurate, they are not precise vertically as to Tail height in the side view, and are also not accurate as to Tail center, Tail height, and Tail width in the Rear view of that trailer; re-size / position as necessary.
When the above Texture image Material is mapped onto
ex: the front side, Select that Face, Right-click, and choose
Texture > Position
* Click and Drag the
Red Pin to
Anchor the
Bottom Left corner of the Material at the Bottom Left corner of the Face
* Click and Drag the
Green Pin to set a position for the
Bottom Right corner of the Material at the Bottom Left corner of the Face
NOTE: If necessary, pull up or down on the
Green Pin to rotate axes / size a mapped Material to align an image along a Face
Edges
* Click and Drag the
Blue Pin to set a position for the
Top Left corner of the Material at / near the Top Left corner of the Face
NOTE: If necessary, pull up or down on the
Blue Pin to tilt axes /size a mapped Material to align an image along Face
Edges
* Click and Drag the
Yellow Pin to set a position for the
Top Right corner of the Material at the Top Right corner of the Face
When the mapped Texture Material is properly positioned / sized / axially aligned with the Face, right-click and choose
Done
BTW: Depending on how much detail you wish to put into this first 3D model, and whether it will be a significant visual asset that user aircraft will be able to see up close in FS, you may wish to add some more details to enhance realism.
CAVEAT: One must always keep in mind that one has a finite "budget" of system resources and rendering requirements that can be accommodated in a local scenery Area by the Windows FS task session's
User Virtual Address Space (aka "
USERVA"), and by the FS run time rendering engine sub-system.
So, we must choose carefully where and how we allocate parts of our "FS budget" to render scenery / aircraft / gauge info in detail.
P3Dv4.x can use 64-Bit MDLs for 3D objects under DirectX-10 and DirectX-11 now, which makes more 3D model and scenery detail possible, so feel free to try out a very detailed model, especially if you may enjoy examining it up close at run time in P3Dv4.
GaryGB
