The best capacitors I have found for this application are the WIMA MP3-X2 series. They are across-the-line rated capacitors with a vacuum epoxy encapsulated paper film dielectric of similar construction to the Rifa (now Evox Rifa) capactors in your construction notes Jon. I have found their sonic properties to be better than any other capacitor in this size range.
In addition, the construction is better and the leads are copper as opposed to the Rifa series. The vacuum epoxy impregnation is of course done to remove all air from the windings so they will not support a fire in case of catastophic failure due to voltage transient. A consequence of this construction is that the capacitor becomes a solid block of epoxy because in the smaller sizes (up to about 15MM PCM) the epoxy penetration into the paper dielectric is nearly complete. The sonic rewards of non-moving capacitor plates can be quite astonishing in many applications. I would encourage others to experiment with these amazing capacitors in all applications.