Openswan CentOS 5.4 public IPSEC to M0n0 behind NAT

 Hey guys, here is my scenario;

m0n0wall is behind a NAT box that we have no access to (so it receives a
private IP on the WAN interface 10.10.20.xxx)

The centOS server is on a public static IP behind no firewall other then
it's own (iptables running on the server).

I want to create a persistent ipsec tunnel;  server <-> m0n0wall .

I have spent hours reading, trying etc. I've got a huge headache is about
all to show for it and most likely screwed my centos config pretty well.

Either way, can someone help out? Is this even possible? The public IP of
the router m0n0wall is behind is static, but m0n0 can only get a private
NAT'd ip.

ipsec.conf examples for the centos box?

Thanks a lot.

-J