Cut the end board off a pallet – free pallets are available in Bunnings timber yards.
Cut several holes to a size that will suit your chosen pots. If you don't have a hole saw, Bunnings has a budget kit.
Use a tool with texture to distress the pallet. I made this distressing tool with some old nuts and copper fittings. Pay attention to the edges when distressing the pallet as rustic pieces normally get more damage there.
Stain the marks you made as dirt usually builds up in crevices. I've used a variety of what I had on hand – spray paint and Feast Watson Oak stain. Rust-Oleum created a helpful video about How to Apply Chalked Paint.
Remove all surface stain and paint by sanding, leaving the darkened timber in certain places such as any cracks in the timber.
The pots should hold in place, but you can use clear construction adhesive or hot glue to fix them in place.