As chsh said, security is a layered process. Or rather it should be. Most of the time it is not. Securing a slackware box is not any different that securing any other nix distro, it's a bitch and is time consuming. Not because it is nessesarly difficult, rather the methods inforced, the deployment of the security if you will. Before any IPtables are made, before kernels are updated. I say this because as I'm sure you all know, one of the first things you are looking to determine is what kind of services you will be running, what kind of risk are you looking at from running these services? Will there be any other users? How many if so? Who might try to take advantage of the situation? How about physical security? What if I have all of my software updated and configured properly then what? Most after asking themselves this question, IMO, would even if in a subconsious form belive that there work was done. They would put off updates and tweaks for months afterwards. By the time they get around to doing any more security focused work they find out that there system has been compromised.
So I believe that securing you slack box, or any one for that matter, the first question you should be asking is not where do I find the latest and greatest firewall or what configuration should my inetd have. Rather I think you should ask yourself; Do I have all my ducks lined in a row? In other words, am I ready to devout time to the security of my slack box? Sounds cheesy, gay whatever, but seriously, why would you just throw a firewall (which I think too many people use as a blanket rather than an additional line of security) on your system and think your good to go? You better have some time and patinence if you are seriously interested in locking down any box and keeping it locked down.

P.S.
For those of you who still didn't catch my drift on why securing any nix box is a bitch I will state it more clearly: Your work is never done.