i built my own.
cut some sections off a 1.5" pipe. split and flattened. drilled holes to fit the 3/4" pipe in the 'squares'
everything was cut according to specific water heater demintions. can't, or, shouldn't, go below the drain or above the relief hole (kept the 3/4" within range) where i Tee'd into...
drilled and brazed some 1" unions to the outer shell.
fitting everything together.
brazed them up.
brazed unions on the 3/4" ends.
presto. no soldering required. wrench off for cleaning, wrench on to put back in service; as water is solders enemy.
i made some 10 footers out of a bunch of 1.5" pieces brazed together to 'preheat' the fresh well water to the DHW, then the standard side arm to keep it circulating....
instant recovery with volume at hand. win twice. 1 pump required, hits the thermosiphon, then the preheat, then HX for house.
forgot to metion, for preheat, OWB direction hits flow from well head on, most exposure to high heat with entire lenght of tube.