So, this project is actually pretty simple and inexpensive. We use washable fingerpaint for ours, and let the rain or squirt bottle do the cleaning afterwards.
Piece of Plexiglass (Lowes has precut ones available, but will also cut the glass for you)
Two boards the width of the Plexiglass (I had some left over plastic composite wood which has worked well in the elements)
Two or four bolts and locking nuts (Long enough to go through both boards with room to spare)
Two brackets/eye screws (to hang from your fence or other surface)
Drill, Clamps, wrench, pliers and (maybe) a Saw
Painting supplies: brushes, buckets, squeegie, squirt bottle
Step One: Drill Holes and Attach Bolts
Start by clamping your boards in a plexiglass sandwich. I didn't have to cut my boards to be the same length since they already were, but you might if you get a new board.
Going slowly, drill a hole through the boards and plexiglass sandwich. You will want to use a drill bit that is wide enough to have the bolt slide through easily. I did four holes/bolts in my boards. Using a wrench and pliers, secure the bolts through the boards and plexiglass. Make sure they are firmly attached but not too tight. You don't want to crack your plexiglass.
Step Two: Attach Hangers
I had some of these metal brackets hanging out from a previous project. It was rather easy to add them to the back of my board through the top set of bolts, but you can easily attach them using a set of screws as well. I added some washers just so they wouldn't wear into the boards over time.
Step Three: Add Hooks
You can now remove the clamps. I added a set of cup hooks to my boards to hang these small buckets for paint. The buckets are from Target. I might also add additional hooks for the squeegie and spray bottle. I slipped some rope through the the brackets and hung it on my fence.