Made sub worse!

[MKZoom]

using duct tape to seal a port should be air tight, which is good. but its not very stiff, which is bad. something like MDF cut to size with epoxy would be "better". but really you can just do whatever works to create an air tight and stiff closure, assuming you are confident sealing is the route you want to take.
i mostly just lurk here but i see a lot of people doing things with partially flawed assumptions on audio.

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[brkdncr]

By the way, how are people sealing the port? Would duct tape be effective?

no, you have to seal it solid. Tape would not suspend the woofer, would also act as a passive radiator, and would blow out in short order.

box size would also need to be tuned for the speaker. You can make the box “bigger” with poly fill or smaller with physical solid mass