We recommend using corning glass crucibles (Gooch, fritted disk, Pyrex® 50 mL, pore size - coarse, ASTM 40-60 mm, Corning® No. 32940-50C) which can withstand 525C.
We are not aware of ceramic crucibles with filter beds ~50mm pore size that would be suitable for filtering.
Any changes to the assay procedure would need to be internally validated.