If you already have a NetBSD host you can also create an image of the
compact flash, customize it to your needs, and then copy it to the
card itself using a USB/IDE/whatever to CF adapter.
Using this method i built a multi-user "shell box" fitting on a 8M CF
(on a soekris Net4801; nothing spectacular but it may give you ideas).
I think the most important point is to have everything mounted
read-only except for /var and /tmp (i used a mfs).
Here is a link that can get you started:
http://bsdsupport.org/toc.php?paper=nbsdcf
There are many other interesting pages about this but i don't have
them handy. You may also want to check in the tech-embed mailling-list
archives.