This is a idea the built into a dream, it is a 24ft diameter galvanized stock tank "bottomless tank" I used two of them and bolted them together with one upside down.
Just warmed up enough to swim in it and I love it and wouldnt have done it different.
LINK:
www.tankandbarrel.com/bottomless-stock-tanks-45-inch-high-bottomless-stock-tanks-c-280_284_686.html?view=all
The Liner is just a above ground pool liner of your choice just make sure its the right diameter.
Will add the other components later: