James Toomey wrote:
We have a standard 30-gallon water heater on the outside of our house,
which used to have a metal enclosure around it.

snip

2) Build a 2-foot footing, then plop a standard-sized enclosure on top
of that.

Probably cheapest and easiest if the enclosure is an off the shelf
item. It would look rather neat if the height extending masonry were
some nce stone or brick layup. HTH

Joe