I’ve got a problem with sagging cables, and I’ve got a simple solution. Examine the side-by-side images below which show the same fiber connection between a switch and a firewall. The image on the left shows a sagging cable which crosses in front of the switch in the rack unit just below it.
As you may know, this cabling install is a violation of the 167th rule of networking:

Thou shalt contain your cables to your own rack unit and shalt not, under any circumstances, impede access to other rack units or blades.

The image on the right ticks the box for me. There’s no room for a dedicated 1RU horizontal cable manager, but there is room for a zero-RU strain relief bar (as seen below). The result is a relatively neat cabling job. It’s no work of art, but it’s functional.
A strain-relief bar is a cheap metal bar that you can bolt on when you rack-mount your switch. It allows you to velcro your fiber patches to the bar, taking the strain to help prevent breaks and preventing the dreaded cable droop. You should, of course, take care to ensure you don’t block access to any field-replaceable units, cards or ports on your network device.
The strain-relief bar is a cheap and easy solution to cable management problems when you can’t dedicate a rack-unit to cable horizontal cable management.