I've never mucked about on the inside of a 900 (is it 900v2?) but the best way when there isn't holes in the mobo tray is to route as many cables behind the drive cage as possible (if possible) and really think hard about eliminating the needs to stretch unnecessary cables to different spots in the case. For example, instead of taking a long psu cable that has both molex and sata connectors on it all the way up the case, just get a molex/sata splitter to add to one that's already located up there. I'm sure you get what I mean because it's hard to explain lol.
Split loom and velcro adjustable cable ties are a must. Loom makes any visible cables look tidier and keeps multiple wires encased inside of a bigger wire.
Also, when you have a black interior after powder coating you should try to use black loom and black sata cables, so they just kinda blend or camo in.
Most important part is use your imagination. A trip to the dollar store will give you all sorts of ideas.
EDIT: Keep in mind the current state of your management isn't bad at all, I just know that there's always room for improvement.
