How to frame a wall in the attic?

[Fritzell]

With the caveat that this might possibly fall into the wrong category, I hereby express my concern and hope that a kind moderator will overlook it.

To the subject. We are going to insulate the attic in our log house from around 1890, and make two rooms and a WC. We planned to place fabric between the rafters and braces (plus sparse paneling) and hire a company to fill with cellulose insulation.
As you can see in the picture, the braces are somewhat angled but we would like to have a vertical wall. Partly because the slanted surface cannot be furnished in a suitable way and because the space is needed to run the drainpipe.



The question is how to best frame here so that it becomes as good and stable as possible. Due to a rather long drainage route and thin intermediate floors, there will be a wall-hung WC, hence the framework needs to be done properly.
So far I have not come up with the best solution and am wondering if anyone can give a good tip.

The yellow lines symbolize the fabric between the rafters and braces and the green lines the vertical wall.

One solution is to nail standing studs to the side of the rafter. Simple installation and easy to get vertical and have the same protrusion on all, but then it will be hopeless to fasten the fabric in a good way, which more or less rules this out.

Another is to cut the studs at an angle at the end and attach them on the "outside" in the crease where the brace transitions to the rafter. This doesn't complicate fabric installation, but it becomes very difficult to get a straight line longitudinally since not all braces are the same, leading to a warped wall and difficulty getting the right angle and a stable attachment.
A third solution I came up with was to cut a stud at an angle lengthwise and attach it between the braces. The problem with that is that I don't have a suitable saw to do this.
How would you do it?

 

[larsbj]

Place a rule on the floor/joist and set one on the rafters, then rule between these two. In this way, it will be easiest to get the wall straight and plumb.