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:
http://www.amazon.co.uk/dp/B004A88WUI/ref=pe_385721_37038051_pe_217191_31005151_3p_M3T1_dp_1
which is mounted to the bottom Magura mounts with some spacers.
Then I took apart an M-Wave quick release bottle cage:
http://www.amazon.co.uk/M-Wave-Universal-Release-Drinks-Bottle/dp/B0015UBI7S/ref=sr_1_26?s=sports&ie=UTF8&qid=1422389251cut
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 
