As Joe said or cut up a grip and use that to "clamp" the raised willow in place. or do what the master does and use parcel tape to "clamp".
Get some watery PVA glue or water some down, pour it into all the cracks all over the bat making sure all the cracks are filled, then bing the whole bat with string very tightly to make sure the cracks stay down. Leave it overnight, take the string off and sand. As seen on Beach Cricket's blog (link)