Keith, there have been some very good suggestions already (I read them all), so I won't go over them other than to comment on the possibility of movement when using the rubberised type mat. A jig is normally designed to allow consistent, accurate work; a mat that has even limited movement will obviate the purpose.
I did notice that non-one has mentioned a vacuum clamp, which would seem perfect for your needs, and reuseable, though you said a clamp was a bit over the top.
A hybrid would have holes top and bottom to allow a generic plate that you can sit things on to hold.
Two sided carpet tape seems like a winner too.