My HLT is built around a Mini-Brew MiniHot Liquor Tank. It is a 15 gallon tank made from high density polyethylene (HDPE) with six 1/2" NPT ports and one 1" NPT port. The only modification I made to the tank is to add a hole at the 15 gallon level to mount a float switch.
Heating is accomplished using a standard 4500 watt, 230 volt hot water heater element mounted in the 1" port. It is driven by a 25 amp solid state relay in the controller and produces about 2 degrees/minute. The control thermocouple (Omega CF-090-J-4-60-1) is mounted through one of the 1/2" ports on the bottom of the tank.
Early experiments showed that the tank was going to have to be stirred by some method to obtain even temperature distribution throughout the water column. I chose to do this with a small pump taking water off the bottom and returning it to the top of the tank. The inlet to the pump comes off a tee just prior to the outlet ball valve and the pump outlet goes to a tee in the lid shared with the water inlet line. The small line going to the lid in the photo comes from a Reverse Osmosis water filter.