Actual internal dimensions used will depend on 3/4" x 1/2" internal corner posts so the usable area is 174mm x 75mm 6.8" x 2.9". But if you can knock out the corners in your project, the usable area is 192mm x 112mm 7.6" x 4.45". Inside height is 1.55 inches to the hem - minus the small risers, add just under 0.5 inches for the inside top. The small risers are 0.2" in diameter but can be sanded flat. Subtract 0.45" from the duct circumference if there are to be small risers.
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