Passive radiators or a plain port for a portable build - what would you pick?

I built almost exactly this two summers ago, a plywood box about that size with a pair of 4 inch drivers and two passive radiators on the back, running off a small amp board and a 4S pack. I went passive mostly because I kept chipping port tubes against door frames, and the sealed-plus-radiators setup has been fine. It does roll off a bit lower in volume, so I picked drivers with a decent cone area rather than chasing excursion, and that made the difference. At picnic volume it sounds plenty full, at full tilt it thumps but only for a few songs before the battery gauge starts sliding.

On the watts question, my experience matches your gut. I put a 100 watt capable board in there and I honestly never sit above what feels like a third of it. The biggest benefit was headroom on bass-heavy tracks, not sustained loudness. If I rebuilt it I would use the same amp and spend the savings on a bigger battery. One thing I would do differently is port placement if you do go that route. A slot port along the bottom edge facing down chuffed way less than the round tube I first tried, and it ate less internal space than I expected.

I went the other direction with my camp box and never looked back. Mine is a ported build with a single 5 inch driver and a little 2x50 watt class D board off a 3S pack, and the port has been a non issue. I cut a flared exit using a piece of ABS elbow instead of a straight tube, and I've never heard chuffing even when I crank it at the lake. Wind noise turned out to be overblown in my expirience, the air moving in the port only matters when the box is genuinely being pushed. The bigger lesson for me was on power. I initially paired it with a bigger amp thinking I'd want headroom, but the battery sag was annoying and I rarely used it. Downgrading to a modest board roughly doubled my runtime and I honestly can't tell the difference at the levels we actually listen at around the fire. Nobody at a campsite is doing critical listening, they're yelling over it for one song then letting it simmer.

The other poster's passive setup sounds fine, but for me the port bought me real output down low without needing drivers with huge excursion, and a port costs almost nothing if you want to rebuild. My advice whould be to mock the port up with a cardboard tube taped through a hole first, tune by ear, then commit. Cheap experiment and you'll know in ten minutes if it suits how you listen.

If you are serious about making a small box sound right, forget the thickness of the wood for a minute and think about what goes inside. A cheap internal damping material, like some foam or even just stuffing some old socks into the unused air gaps, makes a difference that you won't hear untill you crank the volume up. Sealed boxes need that internal dampening to control the driver movement and prevent unwanted resonance from the cabinet walls. It keeps the sound tight even when the bass hits hard. I always found that simple trick made the whole setup feel much more robust.

I always get stuck on how big the driver cone needs to be for that deep bass sound, not just the power of the amp. I imagine if the speaker cone is too small, it won't move enugh air even if the amp is strong enough to push it. One of the builders said they picked drivers with a decent cone area, wich made the sound full. How much does the physical size of the speaker itself really impact the sound before you even look at the power rating? I just want it to feel good when I'm sitting near it. It sounds like the difference between these two designs isn't just loud volume but the way it sounds when you crank it up for a minute. That's interesting.