Right, so this isn’t my current project but I was asked about it in a different thread.
Here’s how my 36er’s rack is put together.
The rack is a mini rack like this:
which is mounted to the bottom Magura mounts with some spacers.
Then I took apart an M-Wave quick release bottle cage:
off the interlocking cross section and drilled a hole in the rack so I could screw the two together.
I needed to put some old inner tube around the frame to stop the clamp marking it and to ensure it wasn’t going to move but that’s it.
The bag I have on has a natural rise at the back which goes against the slight downward slant of the rack making it roughly flat on top.
It works well but it does add a bit of weight. In the summer it means I can ride without a backpack to stop me getting a sweaty back 
