It's best to create a sda4 partition on the USB device, that's compatible
with the original Iomega ZIP drive and is the most compatible scheme with
most BIOS's.

Then you'll need syslinux, syslinux is a boot manager for Linux that boots
Linux from a VFAT filesystem.

It works very similar to Grub and Lilo, so you need to put the kernel file
and the initramfs image on the USB, set the syslinux.cfg file, umount the
USB and then run 'syslinux /dev/sda4'.

I'd suggest you to get the latest version of syslinux from
http://syslinux.zytor.com/ since the version in Slackware 11.0 is a bit
old. You can also find info on making USB's bootable there
http://syslinux.zytor.com/usbkey.php

--
damjan