At first, I thought about re-using the original metal mounting chassis, but quickly discovered that just wasn't going to work.  First, the mounting tabs that originally held the 5-1/4" floppy drive were just a little too close together for the IDE drive carrier to fit (probably not surprising, since there wasn't really a "standard" physical form-factor for disc drives back then)...

...and second, the chassis is formed out of a solid piece of metal.  This meant that the back end, where the USB-to-IDE electronics panel from the external-drive enclosure is intended to go, would've needed to be sawed or cut off.  Modifying the metal chassis to my purposes was obviously going to require some significant metalworking...  And since I didn't have the right kind of tools for that sort of work, I was forced to discard this idea and go to plan "B".  (Which didn't faze me much, really, since I'd already started with the assumption that I wouldn't be able to re-use the original chassis.  Still, it would have been convenient...)

(As a side note – check out that huge freakin' transformer attached to the chassis.  No wonder these drives are so heavy and get so warm when they're running!)

Fortunately, I already had a convenient source of material to make new mounting brackets out of – namely, the "Combo External Drive" enclosure whose electronics I was transplanting into the C=1541 case!  Both it and the C=1541 were made of fairly easy-to-cut plastic, so a small hacksaw made quick work of them both.  (Which is why the cuts are kind of ragged – I could've done much better with an electric jigsaw, or a Dremel cutter, but I didn't have either of those handy.  Besides, once the case is closed, no one will see them anyway.)  Cutting off the whole back end of the bottom half of the plastic shell gave me a mounting bracket for the electronics, and cutting out a couple of L-shaped sections gave me a pair of brackets to hold the IDE drive carrier.  Since there were no convenient places to use mounting screws, the electronics bracket was tacked into place with some 3M Super Glue Gel, then hot-melt glue was used to give some additional structural strength.  The two drive brackets up front are held in with screws.

So far, so good.  At least, until I installed the electronics panel... and then, a couple of minor problems cropped up.