I don't know how to approach this wedge issue

I’m trying to build a sewer and I’m having trouble with wegdes. What I’m trying to do seems almost impossible.

I’m trying to make the wedges go up at a 45 degree angle and I have tried archimedes.

Extend the bottom black Part into the 45 degree light gray Part.
Put a 45 degree Wedge flat on top of it so the angle, aligned to the top corner of the light gray Part.
Put another black Part on top aligned to the flat side of the 45 degree Wedge and continue it across.

It’s also hard to tell from the picture, but is the top light gray Part angled already, or is it just a rectangular Part?

do you mean the higher light gray part?

Yes.
Is the gray part just a step up (squared edge)?
Or is it a sloped up hill at 45°?

The higher gray part is a regular part attatched to a wedge that is (0.1, 0.1, 0.1)
Here is a side view:

So like I said, the lower part can extend into the wedge so it looks like the end is angled 45 degrees.
Add a normal black wedge (.1,.1,.1) to the top and put a (.1,.1,xx) Part like you have.
You can’t just add the wedge at the corner though since it will make that section of black too thick.
Slide the black wedge and top black Part back and forth until the distance looks right (.1 stud thick) on the sloped area.