The entrance to this very small parking lot is on the left side of the photo. Daily charge is 40-50 baht. The attendant is there every time I have arrived/departed so he keeps a good eye on things. He put some of my gear into storage so I gave him a little extra money at the end. There is a bathroom in there as well.
Directions: The lot itself is located as far north as you can go on the first road (small alley) on the east side of Route 1. The buildings at the t-junction are actually along the river bordering Thailand and Myanmar.
Heading north on Rte 1 you take a right turn (or u-turn and go east) about 300-400 meters before Immigrations office. Take the first left and follow that narrow road/alley until you see the t-junction and parking lot is on your left.
Safe parking!