That's pretty much what I do to make odd sized cutouts, and I finish it off with set sanding with fine grit sandpaper. I've never done anything as big as a cart slot before though.
I have never worked ABS as anything other than a car bumper or a piece of plumbing. ABS is easy to work with so long as you don't generate too much heat. Even using a hacksaw too quickly can cause it to fuse back together where you cut. I think you are correct to work it slowly and to sand afterward. Another option which is nearly unobtainable and/or a pain in the ass would be water jet cutting or even simply cutting it in tub of water. You could probably get a "helper" to spray your blade with water just to help remove heat, and work a bit faster. I hate ABS plumbing, and would rather have PVC...or better yet copper, but that is just my opinion based on experience with soil pipe lines.
Also, since this is the clean work thread, I wanted to thank GohanX and Adeneo for advice they gave me on irons. I did get my KSGER, and did a trial run on a random board I pulled from a broken digital outlet switch/timer. I was happy with what I could do with basic small puddles and wire, so I moved on to the real thing. This PSIO is quite honestly the first thing I have seriously soldered. I have an ego, am fearless, and solder and weld (pipes) and this went much better than expected. I was planning on ruining this board, and doing the real deal on my "better" unit. I do not in any way consider this "clean work", because there is plenty of room for improvement. However, I am also very realistic at how small some of the vias are on the board and that most people wouldn't be dumb enough to attempt this with the amount of "skill" I have. Even I figured on total failure right out the gate, so I am quite pleasantly surprised. I used NO hot glue. This alone makes me feel good. This board is from a nearly complete collection including the box and all of the pack-ins. The only thing I am aware of missing is the period-specific demo disc. I may re-sell it for fun and profit, and haven't decided. I will know more when a respectable SD card arrives in the mail.
If I can do a PSIO, I think that with some practice that I can solder anything.