After several months of lingering in my garage, I finally finished my 50 qt. Rubbermaid mash tun. The cooler I used is the one shown below:
I picked it up at my local Menards for about $30. You should be also able to find them at Walmart for about $40. I chose this cooler because I've purchased a few of them over the past years, so I know they'll likely be making them for a while. Also, the design is relatively easy to build a mash tun manifold for. And, according to this chart, I should be able to make up to 10 gallons of wort comfortably in there, allowing me to eventually make double batches.
The manifold is a design that I came up with, and eventually I'll get around to actually measuring and posting dimensions of the piping so you can build one too. I used under 8' of copper pipe for it, and should have enough length left over to eventually build a fly sparge manifold (or at least a start on it). The design relies on two lengths of bare copper wire to hold the whole thing together, and two small segments of tubing at the back of the cooler keeps the manifold securely wedged in position. I'm not averse to soldering, or even all that concerned about contamination from the silver-based solder. My main reason for going solderless is to make it easy to clean. I can literally disassemble the entire manifold into individual components, ready for a thorough brushing if desired.