Be careful about the box you put the samples in -- you need to make sure it doesn't contribute any substances, doesn't average other material into the sample and doesn't make a space between the sample and the detector (with a weak x-ray, this affects the signal strongly).  Measure with and without the box.  My solution has been simple and cheap:  white paper mini-cupcake holders meet all the criteria, fit the window size in the test stand and you can get a year's supply for $2.