I think you are correct…that sounds like a panel resonance. Even if it’s a standing wave at 500hz you would primarily hear it out the port or the driver opening. Ditto if it was a driver resonance.

You could rebuild the enclosure with the same wall thickness and just add a several internal braces to stiffen the walls. You just make it a tad deeper to offset for the volume of the new braces.

BTW there are a lot of great TM DIY designs out there from this website as well as HT Guide forum. Not to knock your speaker, but you can get a lot higher quality driver and crossover components for your money if you are willing to build the cabinets.

Have fun.

Mike