Why not the M203 style swivel that mounts on the barrel under the FSP? BushMaster has a good one, as does Brownell's.
For that matter, Brownell's probably has exactly what you described.
.
this?
$35.
Promag makes one for 20.
Conversely, those blackhawk mounts are 6/ea. I'm going to need 4 of them, plus I can use the bh ones on my 2 AKs as well.
I may order one of thse bushy ones too just to try it out.