If the resulting standard is not going to be calibrated, the tolerance(of even 0.005%) will contribute to most of the errors when measured by a DMM. In this case, no bother to things like low thermal EMF binding post, box type or thermal stabilization for Z-foils.
If you want the DMM measurement down to, say, 3ppm, you must consider the following factors which roughly has 1 to 3ppm effect each in a normal room:
TCR, Aluminium case, Low EMF binding post(100 Ohm only), aging