I think what you've configured is probably the way to go around in FHS-based distro - using config files. I am on Guix (btw), and I already have a swapfile partition configured when I set up the system configuration:

system.scm

(use-modules (gnu)
             (guix packages)
             (nongnu packages linux)
             (nongnu system linux-initrd)
             (...))

(operating-system
  (...)
  (swap-devices (list
                              (swap-space (target (file-system-label "swap")))))
  (...))

Here's the size it was allotted:

$ swapon
NAME      TYPE      SIZE   USED PRIO
/dev/sda2 partition 7.4G 724.5M   -2