@dave13 thank you for being patient with me. I completely missed this. Feel free to tag me anytime in a post. Currently the box design is looking nice. Just a few basic questions. Is the internal volume accounting for the port? AKA the port is not being accounted in the box volume.The bracing looks great. I would take a look at your first port resonance to see at what frequency you are starting to get the resonance. Ideally it will be twice as high as your intended crossover point (ex. 80hz crossover – first port resonance 160hz or higher).
One issues I do have is that the port size you gave is not tuning it to 21hz. When I am calculating three ports 1.5″ x 6″ I am getting a port length of 38.5″. I am not sure where the discrepancy is coming from. BUt I would want to double check it.