Technology

There are white box n100 based fanless devices on Amazon for this purpose. They come with four ports and all you need to do is add a ssd https://a.co/d/09BB3vAJ

You don't need much. A raspberry pi3, or similar, with two ports will do it. 4GB memory is way more than enough even if you're loading all the pfblocker block lists, and 1GB will do it if you aren't.

And it really is nice. You get to do all sorts of cool networking stuff.

Any tiny/mini/micro systen, get one second hand from EBay, get the Allstate insurance option in case it turns out to be a dudd, and slap opnsense on that thing. Most of their energy pull will be sub 15w, many even sub 10w.

HUGE EDIT: Ensure that you get a mini PC with at least two Ethernet ports or one that you can put a nic into, but finding form factors with a pcie slot will be harder. If you need a nic for specific reasons (10Gbit, or fiber, or something else) stick with a mini tower form factor instead of a tiny/mini/micro form factor. Those almost all have multiple pcie slots for expansion.

how low are we talking? I run a homelab with mini pc's and pi

The raspberry pi runs at 3-9W each mini pc runs at 25-65w

Beware tho ... open sense is not stable on ARM, and requires some hacking around to work

A NanoPI with OpenWRT (there is a variant that NanoPIs have that is called FriendlyWRT, but don't use that, it sucks), they have an ARM CPU so the power usage is low. My NanoPI R5C can reach up to 600Mbit/s (up/down) with SQM enabled (Smart Queue Management, to keep your latency down on high network usage), and 800Mbit/s with something called Hardware offloading.

Technically any x86 PC with the ability to have two separate Ethernet ports would work, but unless you plan on grabbing a NetPC from the 2000s/2010s (which will have dogshit processors and probably no expansion slots for the second Ethernet port), you might be out of luck.

Although if there is an ARM based version of the software (or if you could run it through a compatibility layer like FEX somehow), there's tons of low power devices that could be repurposed.

How do you type your username to login?

for a miini PC I recomend the M720q from Lenovo ... it's kind of overkill but it has a PCIE slot which means you can add a network card inside the original case and because it's 16x you can go up to 10G if you wanted to.

I add external 2.5G cards on my other models via the USB3.0 ports but it's messy and then I have to setup LACP on the switch

From a quick look at the documentation, if the hardware has WiFi capability you can configure OPNsense to be an access point as well. Personally, I keep the two separate: I have a small N200-based box as my OPNsense router and a separate access point running OpenWRT for the wifi devices on my network.