Motor run caps are (at least the new ones) metalized polypropylene,
just like the less expensive conventional polypro caps, but I believe
they are oil filled. In any case, they are very smooth, and available
in large values (10-40 uf) for not too much money. If you want
startling detail and smooth, Obligato might be the only somewhat
more expensive route before you succumb to VCaps, Mundorf, . . .

They are big, but that's no big deal in a speaker. I glue in a little
shelf and then use a tie-wrap anchor and tie-wrap. I wouldn't let them
dangle.

Skip