A bonafide old school sheet metal mechanic could probably scratch out a pattern in a few minutes.

Two adjacent sides are almost a regular saddle layout.
The problem is that the miter creates an ellipse.
This ellipse occurs at 45 degrees to the runs of pipe.
So it effects the curve of the saddle on the areas of pipe where its (ellipse)  at.

If you understand how this layout works. https://www.instructables.com/id/How-to-Layout-a-Pipe-Saddle-Cut/
And you can draw an ellipse where the bottom circle is, center lines lined up,  or trace it from the mitered pipe.
Then you can find the points at the ellipse.

Connect the dots best you can.

But what I would do unless it is really large diameter pipe, is to lay out the saddle best you can and eyeball the the rest.
If you have a lot of these to do, spend time and make one fit, then use it to make a template.
At some point the question is, can I weld this gap up quicker than I can fiddle with this fit?
I've had the pleasure of this joint on handrails or something once or twice.  

I'm curious as to why pipe was used instead of square tube or Wide Flange. I'm assuming this is some structure.
Maybe pipe is available but not tube.

Floyd