I've been sidetracked with a lot of little projects lately, and progress on my larger projects have slowed down as a result. Finally, after several weeks of delays, I've been able to resume work on the water heater cabinet. In doing this, I essentially constructed the entire frame horizontally as shown here.
The cabinet is 8' tall, but the longest metal brace sold at the local Home Depot was just 6' in length. To work around this, I cut four 4' braces into 2' sections and braced the pieces together using ¼" hex bolts. This arrangement lengthened the 6' sections by 2' and joined each section together with a 2' section of brace reinforcement secured by six ¼" hex bolts.
Once the 8' sections were completed, the corners were reinforced with diagonal braces.
Right angle brace reinforcements were used for the adjacent corners.
Then I partially disassembled the frame so I could fit it around the water and gas pipes that lead into and away from the water heater. Then I anchored the frame to the wall using 2" wood screws. Everything is rock solid and ready for the fire-rated gypsum drywall.
Once I mounted the water heater cabinet frame to the wall, I spent some time applying a few coats of water sealer on the boards around the sink area of the work bench. After a few hours, the wood was reasonably protected against moisture in the working area. It isn't anything that I'd submerge on a regular basis, but it'll certainly hold up to everyday sink usage for the next several years.