It's not my idea, but you're welcome.
I just googled 'cooler mash tun build' and found a design I liked and did it. I have two of them. Both coolers are mash tuns in the photo. I basically have a 96 quart mash tun when I piggy back them together. I can do up to 20 gallon batches. I've also have brewed two different recipes at the same time, using each mash tun separately.
If I'm making a really high gravity beer that would be too much for my mash tun, I can always split them into the two mash tuns and then add all the sparge water to the top and drain down through both of them.