A pin spanner really isn't the right thing (although it might work). As Merlin63Tr4 mentioned, the real tool looks like a socket with teeth.
Haven't made one for the OD plug yet; but when I do, I'll just stick a cheap socket in the vertical mill and cut some teeth in it. I've used this approach for several other tools (like the socket to turn the nut inside the shift knob on an OD Stag), and it's worked very well for me.
If you don't have a VM, I'd suggest one of those cheap 4" angle grinders from Harbor Freight and a cutoff blade. Lay out the teeth on your socket with a Sharpie (fine point permanent marker) and make successive cuts with the cutoff blade to eat out each tooth.
Oh yeah, I've got one of those pin spanners from HF, and I don't think it opens wide enough. Also too short for as tight as those drain plugs usually are, and too flimsy for a cheater bar, IMO.