Actually, my inclination would be to go for a livery collar.
I have never seen plaque belts with bezel with that shape. Granted, the heyday of plaque belts is not my period of specialty -others with more experience may be able to provide provenance for something of the sort.
But on chains of office, rondel plaques of that sort were quite common.
I think yours might make a pretty reasonable facsimile of the collar of a member of the Order of the Garter.
Memento, homo, quod cinis es! Et in cenerem reverentis!
You can see some livery collars at http://larsdatter.com/collars.htm -- there's generally a pretty standard set of links involved with the various orders (or the Lancastrian esses, or Yorkist roses-and-suns).
There's certainly also the more decorative collars (like the ones Henry VIII seemed particularly inclined towards wearing in his portraits) -- I could see this being done up as one of those.
