National forest land is OK
You could try the National Frorest lands south/soutwest of Mt Hood. I think the USNF office is in Sandy, OR. They were VERY nice when I asked them a few years ago about places to shoot in the MT Hood National Forest. Basically anyplace is OK and they will give you a map of roads and marked gravel pits.
Also, there is the Douglas Ridge Rifle Club near Eagle Creek (southwest of Sandy). They even have 50 BMG shoots every month or week.
http://www.douglasridge.org/
I you go west of Portland on Hwy 26 toward Seaside there are many logging roads on State Forest lands that also allow shooting, I think around mileposts 34 - 24 or so.
I would be careful about using NF lands alone as there have been a couple of apparent abushes over the years of target shooters alone at a gravel pit.
I live in Hillsboro. If you ever want a shooting companion we could try to coordniate schedules for some weekend day. I plan to join the Tri-County club in Sherwood, (SW of Portland)
but every range has some restrictions and occassionally it would be nice to get out in the wilds. PM me and we can exchange contact info if you like.
When i have gone out on the State Forest lands I keep a loaded weapon with me at all times (walking to target area, reloading primary weapon, etc.), either in a hip holster or a rilfe in my hands and I stay aware of my surroundings.