I just did this about a month ago with the same cooler. I made it square with a crossbar using 1/2" copper tubing. Instead of slits, i drilled a row of holes over the entire top of it using (i think) a 3/64" bit.
I also skipped soldering 3 joints. This allowed the manifold to hinge for easier installation and made it easier to remove for cleaning. I'll upload a quick sketch i just made with google docs
I just made an "almost" imperial stout with about 18lbs of grain and the mash went fantastic. Flowed very well and clean. Clean-up was a total breeze too.