Clothespins (You can find a big pack at the dollar store)
A piece of wood the length and width of your choosing
Hot glue or screws
Take the clothes pins apart and paint them and your wood. Once the clothes pins are dry put them back together.
My wood is balsa from Micheal's so it's super thin so I just hot glued my clothes pins on alternating facing up and down. If you want something sturdier you may want to screw your clothes pins into some actual wood.
Attach hanging wire on the back-mine are held up by a hanging wire on each side. Again hot glue worked for mine because the board was so light but you may need to attach it better if your board is heavier.