Pre-fabricated filter adapter to be manufactured of prime G-90 galvanized steel, 18 or 14 gauge as required, meeting ASTM A-653/653M, with welded corners and seams joined by continuous water and air tight welds. Exterior removable access door with heavy duty hasp or locking systems. All areas in contact with filter plenum shall have double wall construction with a steel liner, and an integral 1" filter rack. The Filter Adapter shall be manufactured by Thybar corporation.