By Northwest Montana I assume you mean the area from West Glacier west while staying north of Missoula and Flathead Lake. You are correct, there is not much fishing pressure up there. Be forewarned, there isn't much in the way of lodging either outside of Kalispell and Libby. There also isn't much in the way of fly shops or guides in this area either.
That said, there is some very good trout fishing in that little known part of Montana. The Kootenai follows Hwy 2 from Libby west and has lots of trout. The Hwy pretty much follows the river, so access isn't much of a problem. However, it is a big river and many places the walk to the river from Hwy 2 is rather steep. There are a few guides working out of Libby and one fly shop I know if in Libby that could get you pointed in the right direction.
The Yaak is another nice river that is a tributary to the Kootenai. The Thompson is not bad for smaller trout. It flows south into the town of Thompson Falls, which is about 2.5 hours west of Missoula (1.5 hours if you know the secondary highways rather than following I-90). There is a forest service road following the Thompson, be forewarned, there is a lot of logging truck traffic on it and they expect you to get out of the way when you meet them.
The North Fork (there is a dirt/gravel road following it all the way to British Columbia) and Middle Fork of the Flathead River can be good. These are isolated rivers with no services nearby. There is a small river north of Kalispell that has OK access and decent fishing. I'll let it up to you to find it. There is also another small river that I'll let you find that flows into the Flathead from the south at Big Lake. Again, I'll not name it, but there is a road that follows it.
North of Missoula 30 minutes is the Jocko River. You will need a tribal permit to fish it, and watch for rattle snakes. It is a very good trout river and in one section of it, the Moise National Bison Range is on the far bank.
If you have a boat, the Flathead River below Flathead Lake has many very large, and by large I mean up to 40 lbs., northern pike. There is also some superb northern pike fishing available in the irrigation resevoirs just north of the town of Hot Springs. These can be fished from shore and there are pike up to 40 lbs. in them as well. How do I know, I lived in Hot Springs and caught many pike of 30-40 lbs. in these two resevoirs.
Because there isn't nearly as many options in Northwest Montana as in Southwest Montana or the Missoula areas, there aren't many who go there as a destination.
Missoula to Peck, ID, where the Red Shed is located, takes about 3-3.5 hours if you drive fairly fast and are comfortable driving fast on twisty, curvy, mountain roads. If you aren't comfortable driving fast on twisty mountain roads, figure 4 to 4.5 hours from Missoula to Peck, ID