When you say 'shear' off the rivet heads, do you mean using a hammer and chisel to smash off the rivet heads? Did you place the chisel under the head before hammering - if so, how did you get it to fit (surely there's not any space for the chisel to fit?)
Do you have to shear off the heads on both sides or will shearing off one side will enable body of the rivet to fall through?
How did you manage to do this without damaging your case?
"Smash" is such a violent word, but, yes.
You can sneak the chisel point under the edge of the rivet head, tap gently to work the chisel in until it's resting against the stem of the rivet, then smash away. The rivets are aluminum, so they aren't too tough.
Once you've sheared it off the other half of the rivet inside is easily removed if it hasn't removed itself. You might want to put a piece of tape over it ahead of time to prevent it from flying off and lodging its conductive bad self somewhere undesirable.
By the way, remove your hard drive first
unless you want to test those claimed G-shock ratings.
Regarding case damage, any scrape marks will be on the bottom of the case on the raw aluminum, so are of little consequence. I just checked mine and apparently I managed to do it without any scrape marks.
Here I am threatening a rivet with my not-quite-ready-for-fine-carpentry chisel:
You can also drill them out as mentioned, however you then have conductive aluminum shavings to contend with. Also aluminum is kind of an annoying metal to work with, and using a hand drill to drill out a rivet is likely to result in some jamming and pulling, so you'll probably end up damaging the case hole a bit. Scrapes or raggedy hole, your choice -- no real functional harm to the case either way.
Finally, with using screws to replace the rivets, did you you try this and what screw / thread size did you use?
I didn't try this, but I'd just use the biggest that fit and the coarsest thread size available to help prevent the nuts from rotating off over time due to drive vibration.
If you don't think you'll be removing the cage much, I'd go with rivets. You can see two of them visible in the photo above, along with backup plates (little aluminum washers sold for use with rivets). As the name suggests, normally you put the backup plates on the reverse side if you're riveting fragile material, to provide a solid base for the rivet to compress against. But here I stuck them on the front side to lift the rivet head away from the case to make it easier to remove the rivets again in the future.
I think I've seen a cheap pop rivet tool and starter pack of rivets bundled together for around $10 at a home improvement store. I'm certain you could get all you need for less than $20, even if you had to buy it at Sears. The advantage over bolts is that they don't vibrate loose and you don't have to mess around trying to thread a nut in a confined space. You might find them useful in other case mods or fabrication as well. And they're fun to pop on there.
But... as I mentioned in my first post, you might not want to use either. I think it would be worth trying to soft-mount the entire cage, with or without the drives being suspended.