The standard Mössbauer sample holder is a small Delron disk with a well of diameter 12.7mm diameter and 1mm deep. Powder samples can be fixed in the well by evaporating a small amount of Acetone off the sample while it is in the sample holder or mixed with Boron Nitride or Calcium Carbonate to fill the well. A piece of cryo is used to keep any sample from spilling out. In general smaller amounts of samples with higher concentrations of Iron work best. If the sample is too thick it begins to work as a shield preventing all gamma rays from reaching the detector
In the image on the left, you can see a fully-prepared Mössbauer sample.