I'm using a grain bag (about 24" square) that I picked up at my LHBS before I made my last batch. More than large enough for the 11-12 pounds of grain that day. From what I could see, it will also be large enough for the brew I'll be making this coming weekend (about 17 pounds of grain)... I might pick up a second bag this week, just in case. I'm planning on using some butchers twine to tie the bag(s) off, so that I can let it 'float' around inside the cooler.
I just checked the pricing on a 32 qt aluminum stock pot (restaurant supplier I have access to)... I think I'll use my xmas money to pick one up... That should be large enough to do my full boils for the 5 gallon batch sizes... I'll also have enough head space for adding hops and such to the pot... I think the 24 quart will be too small, and the 40 quart (and larger) are really too large for now. I might even sparge right in the pot, then transfer the mash water into it for the boil...
I've ordered my grain (and hops) from FarmHouse Brewing Supply (just placed the order) for the coming brew day, plus the following, and should have at least some grain left over. I'll only need to get a little grain, and yeast, from the LHBS for the coming brewing... I'm also setting up to be able to wash yeast, so that I'll be able to save there.
At this point, I'm not looking to make the cooler a 100% mash tun, so installing just the ball valve assembly, and keeping what I pulled out of it (it unscrewed easily) means I can always convert it back into a dedicated cooler. Or just fill it with ice for when I need to keep things fermenting during the warmer months. Although it could be a tight fit for what I'm fermenting in for the current cooler. I might end up picking up an old fridge to use as a brew chamber during the warmer months (or even cooler ones if I install a heat source in it)...