Yellow Coticule layers are generally rather thin and fragile. Thus, they are generally glued to slate or Belgium Blue whetstone to provide stability. Carefully sand the back of the stone flat and then glue it to glass, granite, slate or BBW using a waterproof adhesive.
Although some woods like Teak and hard rock maple can be used, it is not ideal as it can absorb water and warp or swell, which could cause the Coticule to crack.
Here's my 2 rescues. The big one is a man made the small one is an unknown and my best Coticule finisher. Found some 1/2 inch G9 at work for backing. Almost the same weight as the original stone. Nice score!