There is a huge thread on xtremesystems.org/forums that people post to with hundreds of cable modded Antec 900s. Although not all of them are specifically for the motherboard you own, you will get plenty of ideas for cable modding and the techniques/tools/supplies they use to cut the holes. Good luck.
To be honest, I dunno. My guess is cuz the 900 is a good aircooling gamer case, has a side window so you can see all the messy/clean cables, and didn't originally come with ANY cable management holes, builders began modding the case right away. Since they already cut a few holes, why not cut a couple more, then flip the hard drives and the psu, and on and on, until you see what's on that thread. I'm sure there's enough on there to at least get you started and give you ideas.
The 902 doesn't have any ventilation holes on the bottom for PSU with bottom fan, that would be the only MOD necessary for it. You can always flip the PSU upside-down (as I did), but cabling might be a bit tighter that way.
You can create two holes as long as they don't touch, then connect the two by using a dremel or a tool called a nibbler both of which I used. The nibbler works okay, and creates less of a mess than the discs from a dremel, but you do need to take your time, so the cutting edge looks even which most of the time can be covered up by some type of rubber edging to protect your wires.
Thanks for the pics and great info. I found a great tool for cutting 2" round holes - its the "Rigid" brand bi-metal hole cutter (home depot) - and I added the drill bit in the center so it stays where you want it following the pilot hole. I made 3 holes. Also I did a series of holes so that I could flip the PSU - i like the idea of the exhaust going down and out (lol). Im so excited - my first build and Im carving up a case. I love it. Oh, I sliced open "window screen spline 0.190" diameter" (lowe's) cost like $4 for 25' - so i sliced it open with a utility knife (or use exacto knife) as its like a hollow tube and used it to cover the edges of my case holes so that the sharp edges dont cut the wire insulation. Slide through one wall, length wise, then careful so you dont cut yourself with knife or the edges of the hole! Also the case and bit get hot when you cut - dont burn yourself. Mine smoked - beware of fire-hazard. Oh end of safety precautions ....Then i secured it with a dab of black RTV - my fav adhesive I love playing with that stuff lol. Now Im ready to seat the processor and my Zigmatek heatsink!!! THANKS EVERYONE!!! You INSPIRE me!
this hole cutter ROCKS!!! Get the one that has the drill mandrel and add the center drill bit!
the drill bit fits into the middle of the device and makes a pilot hole so that your cut stays where you want it.
you use this set-up with an electric drill. I used a plug-in AC powered drill as it had plenty of torque. The cuts went very smoothly and fast! I put two 2x4" wood lengths under the area I was cutting to support the case (and not drill holes in my floor lol)
I used this with GREAT SUCCESS - nice round holes!!! I already finished. I can do a 2" hole in 2 minutes max. Very smooth. Highly recommended. I shopped around and liked the Rigid the best for the price. You need the version designed to cut METAL. Dont get the one that only works on WOOD - they look the same but are too soft. Mine said "bi-metal," RIGID-brand. I have no association with Rigid or Home Depot, other than giving them my hard-earned money lol