do you have access to reasonably cheap sheet steel (1/4inch or so) from the Houston shipyards?....portable "shoot houses" are constructed of 4'x8' steel plate and connected at the corners with a sandwich of inner/outer angle iron

there are several seawall companies here on the west coast of Fla that make precast/prestressed seawall sections... they are tongue and groove, but you might find someone that would produce an easy to erect out bilding if you choose to not modify existing structure

the easiest, and probably most accessable for construction would to add two more block walls (with floor/ceiling vents) to an existing corner of your garage... hire a concrete pumper and try to vibrate as soupy a mix you can down the existing walls, rebar and solid pour new walls, add a steel inward opening door.. make a monolithic pour to the reinforced roof and lintel... add some bench seats...commode would be a nice touch