Haven't tried the Peerless SPK815 that you're considering, but it may be your best bet. I don't have any faith in the Omnimounts except for use with the lightest of bookshelf speakers; they tend to slip unless tightened to the point of breaking! (I've broken one myself.)

Here's something I found in the Markertek catalog:
[Link: markertek.com]
It's not intended for speakers, or even for wall mounting, but appears as though it may suit your application.

Good luck.