We spent a little time in that area a few years ago. Like you it was late in the season, but we found some great boondocking spots. One night we stayed in a small old quarry ( 40.875573, -109.463932) about a mile and a quarter south of Firefighters Memorial. Another great spot was Stateline Cove ( 40.999641, -109.605988), on the north side of the reservoir, literally right on the UT/WY border.
It's a beautiful area - enjoy!