The off-leash park at the back of the Fuller Lake Arena Grounds is a little overgrown and doesn’t really look like a dog park anymore, but it is still used and does have a garbage can and poop bag dispenser which appear to be kept up. The easiest way to access this area is to park at the back of the arena parking lot, on the left/east corner.
You can also park at the south end of Cottonwood Road and hike up to the off-leash park through the forested trails, which are also off-leash, on the east side of the arena.