I have wanted to make a spencer for years now, and I finally got started on one last weekend. I have a couple yards of black cotton velvet in my stash that I'm going to use for this. I pulled out a pattern I already have, the Period Impressions one.
The mockup is decent, but I haven't had the chance to get my stays on to fully try it out. The sleeve was a disaster at first, but I realized that I had to rotate it a bit to get it to lie right. And I had to shorten it 5 inches. I have no idea why it was drafted so long.