Looking for a small form factor PC to use as a router

I'm looking for a 1L type PC, with dual gigabit, or a single gigabit (or 2.5g) port and space to put in a PCI card with more ports on it.

Planning to use it as a router, not decided on which OS yet, probably OPNSense.

Any advice on what to buy? Don't want to spend more than 150GBP, don't mind buying second hand either.