Advice on a box and removable port design

[Triticum Agricolam]

If you want to tune lower I'd just have the port wrap around towards the front of the box. You shouldn't lose any performance and that would let you tune as low as you want.

You can usually play about a 1/4 octave or so below your tuning before things really start to drop off hard. It kinda depends on what your goals are though. If you are more concerned about SQ than SPL I would tune to the lowest frequency you want to be able to play. You will be giving up some output though compared to a higher tune and your mid-bass need to be able to reach down low enough to give you even response.

For bracing I'd definitely put some pieces running from the baffles back along the port a few inches to keep the baffles from flexing next to the port opening. Ideally I'd like to see something that connects the side pieces to each other and the back, but that's going to be tough to do.

[Woofer Pressure]

Awesome, I'm going to start building today!

[Woofer Pressure]

I've finished cutting all the pieces for the box and it's ready to assemble. Now I'm just trying to find the right way to brace it.

Does this look like the kind of bracing you were talking about Triticum?

Also, are the regular cheap pine dowels enough or do they need to be some sort of special wood?

[Triticum Agricolam]

Yup, that looks pretty good.

And yes, regular pine dowels will be fine, though hardwood dowels would be better.