Vicomsoft's Softrouter Plus is obviously a router, but also has firewall functionality. I don't know how viable it is as a firewall product, but it was reputed to be a robust MacOS product, AND it runs on (some) 68k and ppc hardware. I doubt that you'd get much mileage out of it on a compact (if for no other reason than that ethernet is surely needed), but maybe an SE30? Certainly a Quadra or an old Nubus ppc should do the trick.
I had it around here once upon a time.... Maybe still do. I seem to recall that it was "optimized" for ppc, and this may mean 68040 or higher, in keeping with other software trajectories of the time. But you can hunt down these details.