Before we proceed, I want to take all the vertical distance values for each of the edges (the ones you found in the previous step) and multiply them by two. The values you get is what you'll use in this next step. The reason for this multiplier is simply due to the way shear tool works.
Select the Shear Tool (SHIFT+S), select the layer of one of the edges of our reflection and click anywhere on the canvas.
The Shear Tool dialog will pop up. Leave the "X" value at zero, and for the "Y" plug in our vertical distance values that we calculated and multiplied. Depending on how the side is sloped, you might have to make your "Y" value negative. For my left edge, I'll use a positive value, and for my right edge, I'll use a negative one.
Here's my canvas after shearing:
Note: if you have a good eye, you might prefer to avoid the math and simply click-n-drag with the shear tool to attain proper angle for the edge of the original object to tightly match its reflection.
Note: depending on your object you might find it hard to have the ends meet while shearing. If that's the case, give the Perspective Tool a whirl.