Like the hot liquor tank, this is a regular old half-barrel keg with the top cut out. The couplings, thermometer, ball valve, etc., are the same, too. 

I agonized over the design of the mash tun. I considered a stainless false bottom like the one I use in my kettle until I brewed a few times with a friend that uses one in his mash tun. It's fine if you're just going to do infusion mashes, but if you raise the temperature with a flame, you're going to get scorching. Bank on it.

I considered using a SureScreen™ or an EZMASHER®. I did like this idea until I thought about the abuse it would take from the big wooden paddle I used to use to stir my mash. I e-mailed Al Korzonas to ask about his experience (he markets the SureScreen™), but never got a response. I took this to be as telling as an indictment. (Later, I learned that he was unable to answer e-mail during that period of time, so my e-mail probably got lost in the mix.) 

I thought about asking Jack Schmidling (the EZMASHER® guy) the same question, but while perusing his pages, I learned that he uses a stirring contraption of his own invention which alleviates the problem that concerned me. I took this to be further evidence that banging on the screen thingamajig was a bad thing to do and I'd have to replace it repeatedly. 

I finally decided to use the rigid 1/2-inch copper pipe manifold you see here. I could bang away on this with the wooden paddle and it would never get a scratch. The downside to this design is the hacksaw hell you go through to get all of those kerfs in the pipe, but once that's done you've got a durable manifold. I used the holder part of a pipe flaring tool to hold the pipe in a vise while sawing on it.

As far as the size of the square goes, there's no rocket science going on there. One of the long pieces you see was left over from some other thing and it got collected up for use here. It was this chance happening alone that dictated the size of the remaining pieces.

All of the pieces are soldered together, except at the tee. This allows the manifold to hinge there, so it's up and out of the way for stirring, during the times I'm ramping up the heat of the mash.  After raising the temperature for mash-out, I simply lower the manifold down to the bottom of the tun, then wait 10-15 minutes for the mash to settle before beginning the recirculation and sparge.