"Looking east of there, there is a mesa that at one point comes within 2.5 km of Spencer Gulf and is in excess of 200m high. "
"The suggested location for the pumped hydro is given as Cultana in the army training area. The hills there rise 300m within 2-3km of the coast. "
"There's nearly 300m elevation 2km inland near Commissariat Point."
You need to measure the head from the base of the storage/valley, rather than the highest point.
A seawater pumped storage reservoir would be a completely artificial 'turkey nest' reservoir built on top of the hill using equal amounts of cut and fill, rather than finding a natural valley where you can dam an existing river.
2GL equates to a cube of water 1.25km in each direction. Or a square pool of water (say 100m deep) 4.5km by 4.5km
A 2GL storage built atop a mesa or ridge line doesn't sound real practical to me based on those dimensions.
That's because you've somehow mucked up your maths and ended up being out by three orders of magnitude, just like you were with your attempts on the previous page. Please stop trying to do maths until you've learned how.
A 2GL reservoir would be 126x126x126 metres if it were a cube, or a circle of 160m radius if it were 25m deep - but in practice larger than that as the sides would be sloped. That's small enough that a canopy could be built over the top if evaporation is an issue.
But don't just take it from me, read the Melbourne Energy Institute report linked earlier in the thread which even includes photos of existing 'turkey nest' pumped hydro storage reservoirs. The Yanbaru seawater pumped storage dam (0.6GL, 188MWh, 30MW turbine capacity) is 252m across at its widest point with quite shallow sloped sides, and 23 metres deep. For a ballpark comparison (literally in this case) with other infrastructure in South Australia, that reservoir covers a footprint a bit smaller than Adelaide Oval including the grandstands.