Square for blocks and ribs

Having a support for a carpenter's square is handy when it's time to check the squareness of the cuts in corner blocks.
I made this jig for my Stanley square out of some pieces of a blue HDF board (high-density particle wood).
Board thickness is 12.5 mm (1/2"). This material is easy to cut and file and it's perfectly flat.
The square just slides in and it can be placed on the mould surface just over the blocks.