Ash, One piece, necked down at the middle into a thin band, steamed or boiled, and bent into the shape of the dividers. The legs could be nicely shaped and lightened up, light and wieldy. I think you have the pins worked out. The locking mechanism?
Do they need to close completely? I suppose it depends on how thin the middle band is and if the legs need to come completely together.
A while back I made up a compass as I described. I used cedar, and drilled small holes in the feet in which I screwed in broken off screws for the points. To hold it in position I tack screwed a scrap of sticker onto the set to hold it in any desired position. It works very well, the cedar is on the weak side, ash or maple would from a more ridged set.
I have a picture but am having technical difficulties.
