You are looking for a medium sized waterproof box in the alley. It is wrapped in a black bag for camouflage purposes, do not mistake it for garbage! It should be large enough to hold most trackables.
You will find a log inside. Bring your own pen, though.
Please make sure to close the container well after use, as the massive rain in Bangkok is fatal to log sheets otherwise.
There are some metal edges in the alley. Please do not cut yourself while searching.
If you're looking to live it up, your left hand is near your goal.