Sorry, I forgot. Crummy free help
Here you go.
I recommend using high grade plywood (birch, etc) for fabric track shims. The cost of ripping the plywood will make it less expensive than dimensional lumber and you get perfectly flat surfaces, no warping. MDF OK too but not as friendly with staples IMO. In this case I'd use 3/4 plywood, and 1/2 MDF with the corner eased to your liking. I screw the bottom layer to underlying structurals, then use a 18g 2" brad nailer for upper layers.
You can dado the shim so it fits over the nail lip of the track, but that's more work. Adds support for the upper shims but is likely not necessary. Option B lets you use black track which is more likely the color edge you want to see (not see). The top load only comes in natural color. Don't forget to leave a 1/6" gap for the track in option B. Otherwise it's a pain to get the fabric in there.
Option A gives you a more solid backing, which is only important if things are going to be pushing on it. Which they might in a doorway. You can paint the lip of the natural track in option A, and it takes better than one would think.
Note the natural color track is more brittle than the black and easy to blow up near the ends with the staple gun.
Either of these will allow replacing the fabric as none is sandwiched (under rolled) between the track and the wall. A practice to be avoided IMO.
CLICK on the images to see them full size. I didn't draw optin A as wide as you should build the shim. recommend 1.5"-2" wide on 1st layer if going against drywall....