> Then perhaps, you could just use OPT1 and OPT2, or LAN and OPT1  for your two interfaces and leave
WAN (and maybe LAN) disconnected.  It should work, but I think you would still need to have physical
interfaces to assign to WAN and LAN; I don't think m0n0 will function without LAN & WAN assigned. 
It's kind of a hack, but it should work  that way.  Of course, you're limited on the # of interfaces
you could use in that scenario.

Yes, If you have only 2 interfaces, I don't know how to assign the
second interface as Opt1. The manual config ever ask for lan and after
for wan. You can not cancel the wan and go to the opt1 option if you
have only 2 network cards.

> It could probably be made to work, but since you're only doing very basic routing and some
firewall rules, it would be a much better solution in your case to use something else.  A stock load
of just about any *nix should be able to do everything you need.

Perhaps, but I like monowall. The design, ...