You should be able to get an optician to replace the normal adjustment lens in the external diopter lens holder for one with you astigmatic correction as well. This should help to have the viewfinder image looking the correct "shape". Also when it comes to the diopter adjustment to get the best results from the viewfinder, it should be adjusted to that you are effectively focusing on infinity when looking at the focus screen. It is the image of the focus screen that the camera diopter is designed to adjust. To ensure that you get the best possible view of the screen follow the following system.
1 Remove the lens from the camera, and stand facing something that is nice and bright. A window with lots of sky(as long as the sun is not directly visible) would be best. You do really need to see off to a distant horizon.
2 Looking at the distant horizon, so that you start with your eye at around infinity focus. Bring the camera to you face, then quickly look into the viewfinder and look glance at the AF indicator squares. Don't stare at them and only look for a couple of seconds maximum, before looking away from the viewfinder.
3 Adjust the diopter focus adjustment a bit, say half a turn. Repeat step two, and:
3A If it seems less focused turn the adjustment a whole turn in the opposite direction.
3B if it seems better continue adjusting in the same direction in small steps.
4 Once you have gone past the point of best focus, move back in even smaller steps. This way you will go back and forwards around the point of best focus, until you arrive at optimum focus.
By doing it this way, with lots of looking away from the viewfinder you tend to keep the eye at infinity focus, which is a generally relaxed position for the eye. The eye will not have time to refocus. Doing it without the lens ensures that you are really looking at the image of the focus screen at not at something being focused by the camera lens. The camera lens can really throw you off doing this process, if you have generally poor eyesight.
This is the same process I would use in setting up a telescopic sight on a rifle, that also relies on focusing a screen (the cross hair) with one lens, while focusing an image of a subject onto the same screen with another lens.