For those who don't read Japanese, the procedure is pretty simple.
I had taken off the floppy drive cover, but I didn't need to. I just needed to unscrew it so that I could fit it into the hard drive bracket with the floppy drive cover on (without the screw heads getting in the way). You just place the floppy drive with cover (but without the screws) into the hard drive bracket. The floppy drive cover has some bent metal on the side, so it keeps things snug. There are no places to put screws to secure the floppy drive into the bracket, but it seems to be enough snug as is.
So most of my problems were solved by this guy. There are some amazing people out there that the English speaking world just doesn't know about due to all of our search queries being in English. All we ever see of the Internet is the English stuff, but the Internet is truly global. I need to send this guy a big thank you after the project is done (so that I can send him pictures of it completed).
Now my only remaining issues are that I don't have another floppy drive cable and I have no experience putting holes in plastic in a professional way. I've only hot-knifed things, and they didn't look pretty afterwards. It'd also be nice to see how the original dual floppy Macintosh Portable did the upper floppy disk mount. Thanks all for your help.