Knee Wall Without Top Plate
The normal way to do this is to install blocking between the rafters (for the ceiling drywall backer) and between the studs (for the wall backer). You don't need to toenail all of them... you can screw about half of them.

Easier from a building perspective, run full length 2x4s (or equivalent) top, middle and bottom of the wall (assuming it's under 4' -- if it's taller, then 2 middles). Run 2x4s on the underside of the rafters on 16" centers. You've probably figured out by now that this is a ton of lumber with the added non-benefit of less habitable space. 

Neither of these options require bevel-ing. A tiny gap at the edge of the drywall doesn't matter.

If the underlying problem is the cutting, rent a miter saw for half a day and cut your blocking in one swoop.