Ok, I've taken everything into consideration. I had a general aversion to plastic pieces, and a general aversion to paying a metric ton for an Ortlieb bag because it's a brand (I firmly believe it's just branding, 120$ is steep for a bag that touts nothing but "Waterproof." Ziplok bags are waterproof.) I've reconsidered the plastic pieces, but I figure if I pay less for the bag, a cracked mount will be less of a big deal. The worry that the whole thing will come flying off was mitigated by some common sense- zip ties etc.
So, in the end, I'll be using a dry bag after all.
Stage I:
Cannondale GoPod Handlebar Bag
This is a $70.00 bag on sale for a mere $25. I like that. It's also already made of molded EVA foam, which'll insulate in the winter and pad from vibration all the time. It also has two D-rings on it, which will let me tie it on to my bars as a backup to the plastic mount. However, I am possibly LEAST satisfied with this and wil likely swap it for something else (
This perhaps?)
Stage II:
Neoprene DSLR Sleeve
Here's the main protection from bumps, scratches, etc. I'll pack around it if necessary to keep it in one place. The Cannondale has a neoprene bottle sleeve, but it's unclear what the size is, I may be able to nestle the camera there within this sleeve. I originally considered the
Haven, but the dimensions were wrong- it was too tall.
Stage III:
Dry-sack
I'm spending $17 here to get an e-Vent 3 liter. I figure since the e-Vent allows air to leave the bag, there'll be no condensation from heated moisture inside. I will test it in direct sunlight with a basin of water or something, but I anticipate no problems.
Total spent is $17 + $12 + $25 = $54, versus $120 + $15 for an Ortlieb camera system. I think it'll do the job, but I'll keep the Ortlieb as my Plan B if this setup tests negatively.
PLEASE let me know if you hear any bells ringing in your head. I would hate to discover a fatal flaw when I'm on the road. I tried to give myself multiple failsafes.
One question: I'd love to put a rain cover on the outside of the Cannondale but I can't find one. Suggestions?
EDIT:
Mountainsmith makes a cover for a lumbar pack that has a drawstring. It ought to do.