@ Andy ... to run from usb, you don't have to use make_iso -- just copy any modules you want to add to /slax/modules on the usbstick. You can do that at any time, then just reboot to add them to the running system.
From ground zero, you'd
-- format the stick, probably fat32 (not ntfs)
-- copy /slax and /boot subdirs to the stick
-- cd to /boot on the stick and run either bootinst.bat or bootinst.sh to make the stick bootable. You'll probably have to be administrator to get the OS to allow you to write to the stick's mbr sector.
The only people who need make_iso are those planning to burn to a new cdr.