I have recycled 12″ x 12″ ceiling tile to use as a blocking board. The front (good side) of the ceiling tile has a painted surface which is very hard.
The back of the tile, however, is a softer surface to pin into and because the ceiling tiles interlock, I can create any size blocking board I need.
I cover the ceiling tile with a towel.
Then I lay grid interfacing (sold at fabric stores – it’s interfacing weight and my version has dots at 1″ intervals) over the towel. The grid helps me keep my garment true and makes blocking to a specific size very easy.