You can do full passive, but you need a quite clunky heat sink.

My home router is a $80 N100 + 4x 2.5GbE box, passively cooled.

Where did you buy it? Most of the router boxes I've seen are closer to 150-200?

Ah, bought it while I'm in China. Just checked Amazon and even aliexpress and you are right, it's 150-200. Didn't realize it could differ by 2x.

The article shows a related model with a passive heatsink bolted onto it. The heatsink roughly doubles the volume of the thing, but passive seems possible. If nothing else, the spec sheet says it can go up to 60C.