What about this idea?
Make whatever reservations you can and
pay for them.
http://www.glacier.nationalparkschat.co ... =10&t=9779
This would be a "backup" option if hitching to the border from Waterton fails.
1. Fly into Calgary the day before your trip begins.
2. Drive to St Mary and camp overnight at Johnson's (~$30.00/night and they do reservations)
3. Be at the St Mary Visitor center bright and early the next day to pick up your permit.
4. Drive to Waterton and drop your rental car. Unless delayed by breakfast, or crossing the border, you should be in Waterton by 9-10AM.
5. Start
immediately looking for a ride to the border.
6. If you
are able to hitch a ride to the border, call the shuttle and cancel. Yes it'll cost you but look at it as insurance.
Worst case is you shuttle to the border. COS is pretty much downhill then mostly flat.
So even if you start at 4PM, you should make it by dark...barring bad weather.
I think
Teapot57 would know about driving times from Waterton to the border....and St Mary to the border and Waterton.
There is a LOT of traffic in and out of Waterton. Hard to imagine getting a ride to the border would be a big deal.
Small risk but having you car in Waterton might be worth it.
pete