Hi Conor,
Making a scale Zeltbahn isn't that hard, you just need the right tools and a bit of highschool geometry, and you're good to go.
A Zeltbahn isn't exactly triangular. It actually has 6 sides, like a triangle with the points cut off.
This is quite clear in this picture, copied from
www.mp44.nl
Now to the construction of the Zeltbahn.
First step: Tools:

I took the dimensions and scaled them to 1:35.
Draw a line, 77 mm long on a piece of white paper, make sure to mark the middle of your line.

Measure 70 mm with your compass

Put the point of your compass at the start of your 77 mm line and draw an arc.

Put the point of your compass at the other end of your 77 mm line and draw a second arc that intersects your first arc.

Connect the ends of your line with the point where the 2 arcs intersect. You should now have a triangle with side lengths 77 mm, 70 mm, 70 mm.

Connect the middle of your first line with the "top" of your triangle.

We now need to measure where the head will go through. Measure 22 mm and 33 mm up from your "bottom" line. Mark these 2 points. This is where the hole for the head goes.
(ignore the other little lines I drew for now. I'll explain them later)

Put a piece of crepe paper over your drawing and mark the corners, and the middle of the bottom line with a pencil.

Use your ruler and connect the dots.

Mark where the head goes.

As I mentioned earlier, the Zeltbahn is actually 6 sided, so we need to know where to cut off the points of the triangle.
Starting at the top: The piece I cut off is approximately 2 mm long, and extends equally to both sides of the center line. The right and left corners are approximately 5 mm long and extend perpendicularly from the bottom lines until they reach the side lines. This should give you a bottom length of about 71 mm and side lengths of about 60 mm. This fits well with the real dimensions of 2500 mm and 2100 mm.

Soak in a mixture of white glue and water and fit it to your figure. If you really want to go all in, remember there are 11 holes and grommets along each of the side edges.
Hope that helped.
John