In General, I have had good luck with clock works from Clockit and Cherrytree. They are usually 5/8" thick and 2 1/8" square. You may want to consider using 2 pieces of 1/4" wood, cut out the dimensions of the clock on the back piece with a scroll saw and glue the 2 pieces together. That will give you some recess. The mounting hook is usually flush with the back of the clock so you will need a little room for the wall hook. This may make sense if you are not concerned with the side appearance of the wood. Regarding the shaft length, the external portion of the shaft is threaded for mounting so the length should be the thickness of the wood plus the thickness of the nut and washer and any dial you may use. You can always use a washer/s on the inside if it is slightly too long but you will lose some of the depth if you do that.