I prefer to pre-finish some parts of my projects before assembly. But narrow boards can be difficult to stand on edge while applying a stain or finish. The solution I came up with is the “H”-block supports shown in the photos here. The design allows these blocks to support a board of any thickness. And since the space between the legs is the same as their thickness, they can nest together to hold even the thinnest stock. When you’re done, they stack together for compact storage (inset photo).
I rounded over the edges of longer lengths of wood before cutting them to size. The ends were eased before screwing the parts together.