I used two 4" x 4" 1/2" thick Corian solid surfacing samples which I got free from work. I also made an earlier version out of wood, but opted fro the solid surfacing material so I wouldn't have to worry about the wood cracking when the mount was tightened to the seat rails. A strong, tight-grained wood would work well.
1/4 -20 short bolt for securing tripod mount to seat mount
1/4" clamping bolt with washer, lock washer and wing nut
GoPro tripod mount (see my other GoPro Instructables, I use this mount for a lot of things) OR a GoPro flat sticky mount
(If I was to make another out of wood, I would use a piece of 3/4" wide x 1/8" aluminum top and bottom to spread out the load of the clamping bolt so as not to squeeze the wood too much and crack it.)