What kind of two-sided tape?

[ken cierp]

I use the thin foam gap filling double sided stuff -- its a PITA to remove. But it sticks like crazy and I am confident the small parts (bridges primarily) will not move during the CNC machining.

[Dave Bagwill]

This was quick and easy - a strip of masking tape on the bench, sticky face down, a strip on the small piece of spruce that I wanted to thin, and a dribble of Krazy Glue (craft glue from dollar store, three tubes a buck, this is not thin CA) and voila!
There was no movement at all, rock solid.
The method is good for this type of thing at least.