wrote:

...the mount's minus one of those little stupid
o-rings.

Any ideas on replacing this so the mic sits square in the mount again
as it ought?

Find someone who had an O-ring assortment, like an auto supply store,
machine shop, some place that rebuilds pumps and compressors, a real
hardware store, maybe even a Lowes... hell, you're in N'Orleans, there
might be an O-ring supply house within five miles just waiting for you.
Take your mount and mic with you and try 'em 'til you find one that
does what you want. I'd bet you get away with it for less that five
bucks plus gas.

Or
http://www.allorings.com/
http://www.oringswest.com/

try sizing one already on the mount that matches your missing ring.
These places might only sell you 10 or 20 minimum quantity/order, but as
you have experienced it might be good to have spares.

O-rings are also handy for holding those foam ball windscreens on smooth
mics, you put an O-ring on the mic that fits snug so it's not blocking
or near any openings and push the windscreen over it, the o-ring gives
something for the foam to grab. I've done this with KM-84s.

Maybe you need an assortment. No, I'm not an o-ring salesman.