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Pros often go with 14x14 or larger plastic panels for water heaters to allow good reach for valves and drains. Plastic is preferred over metal because of constant moisture and no corrosion risk. Look for heavy-duty, water-repellent models. One commonly installed is this tough plastic panel https://www.amazon.com/Access-Panel-Drywall-Plumbing-Durable/dp/B085617TBL?tag=contractorhub-20 (check available sizes) built to handle plumbing environments, textured white finish that paints easily, removable door for full opening, and simple no-frame install. Keeps the closet looking neat while meeting code for easy maintenance access.