Box bracing feedback please

So the box is 33 X 25 X 15.75. Inner dimensions are 31.5 X 23.5 X 13.5.

What I'm gonna do is run a 3/8 threaded steel rod lengthwise and along the width as well from the center of the panels. I'm gonna use washers, lock nuts, and bolts on both sides of the mdf panels to secure the rod and prevent air leakage. Considering adding sound deadening around the holes as well to ensure a tight, vibration free seal. Top to bottom I'm adding a brace at each 11 inch interval of 2x2x13.5 inch hardwood. This should pretty much remove almost all vibration and fortify the box for a long time. There will be a nut on the outside of each box face so it'll look ugly but should be totally functional. Any constructive feedback on the effectiveness of this plan?

I don't see anything wrong with that plan you may be able to then the rod further out of the face and make that a functional part of holding on a grill.

Well the rods are being placed on the sides without subwoofers. I'm gonna cap the bolts on each side with some sort of plastic cover for safety reasons.

If the steel rods are thick enough (and it sounds like they will be) then this might work. You could end up with a fatigue area around where the washers are which might loosen the joint over time as the wood deforms a little bit under stress. Or not? You might want to go with large fender-style washers if you can.

If the two rods cross eachother, you might want to tie them together with wire so they don't rattle against eachother.

I'm curious why you chose rods instead of adding more wood to reinforce the faces? Disassembly maybe, but you're gluing braces already somewhere else.

I decided to use rods to preserve airspace. The box is going in the trunk of a 2001 Nissan Sentra and 4 CT sounds 10s are going in it so I'm maximizing the space as much as possible. With this I'll get 5 cube net. And yeah I'm gonna use the biggest washers possible, wire the cross point together and cement it with liquid steel to keep the rods from bending as much as possible.

The reason that I'm not using a rod from top to bottom is that if I screw a nut onto the bottom the box won't sit flat. I thought about it and I couldn't come up with a better idea than just using dowels. And I can't leave it without bracing because the top is where the subs will be mounted.

This way I'm using less than .2 cube of box space. It'll be important when I decide to upgrade. I'll have more upgrade options with 1.25 net per than with 1

Rather than a threaded rod through the enclosure consider using 90 degree triangles on the sides and glue them in, make sure they come 2/3 up the enclosure and it will stiffen the ends up no end, 12mm is fine for it as it’s strength that’s required is in the plane where it’s strongest, you can also hollow it out with windows cut into it so it takes up less space

also rather than 2x2 posts, consider buying a sweeping brush handle and use that screwed in the centre from both sides between the subs, 2x2 is a big size just for a brace that needs to be strong in one direction And take up more room in the enclosure if that’s a concern as you voiced

Sounds like a good idea I'll look into it for sure. The reason for the 2x2 posts is that I don't want the box sagging or flexing because of the subwoofer weight and air displacement. There's 80lbs in subs going in there and later when I want to upgrade the weight will increase to either 108lbs or even 172lbs depending on what I choose to upgrade to. And I can make 2x2 posts from the leftover wood from the cut sheets.

Closing