To me it looks like the right side is convex, which is a little harder to make a plug for. than just a straight wedge. Maybe it's an optical illusion in the photo. But if it's real, you can fit a plug by chalking up the place it has to fit in and rubbing the plug blank against it. The chalk will transfer to the areas on the plug that need to be cut back, Whittle that marked high spot off the plug with a knife, gouge, or sharp chisel, as needed, and try again, Keep taking off the high spots until you have a good fit.
Since this is a wedge shape it's tough to do both sides the same way when one is convex. In that case just fit the right side and don't worry about the left. Once the right side fits the wedge will likely have a gap on the left. However that left side looks straight, so it's easy to make up a wedge for that side to fill the gap betwen the right side plug and the left side. You'll end up with two wedge shaped plugs, but they should fit really well, in fact the right side will probably lock in place when the left wedge is driven home. Hope that helps.