OK, I found a little more info on the one you are looking for, it is actually on a map I have. You're gonna owe me one, maybe, LOL!
I know for sure that the site has been visited by the people who drew my map, and they are usually very accurate about locations.
In the text they place the site as being near Army Hill. It shows on the map as being on the old indian trade route that went all up and down the oregon coast. I see using topozone that there is a grassy area, or more stable, higher ground, right near Army hill. My best guess is it would be in this more stable area rather than the actual dunes themselves, but ya never know.
http://www.topozone.com/map.asp?lat=43.70222&lon=-124.16722
Just in case you're interested, there are 3 indian village sites due south, across the Umpqua, and two more due Northwest on the same side of the river as the fort (north side).
Keep in mind I have never been to the area, so I am going strictly by the mapmakers info, and again, I am pretty sure they have visited the site. PS-The red cross is the center of the map, not an "X marks the spot" , LOL!
I hope you find what you are looking for, HH