Solar storage tank.
Your existing electrical hot water tank is perfect to
go with the home build heat exchanger. By removing the electrical
elements (upper and lower) and attach the heat exchanger to those
holes you have constructed your own solar storage tank.

In addition, a cheap way of making a solar storage
tank is to wrap 5/8" copper tubing around your hot water
heater (off course, take first the enclosure of the tank) and
you have made yourself a nice and cheap “solar-hot-water-storage-tank”

Or go for the professional tank (expect to pay
$900 or more) see picture below.