I have never quite seen the point in making an heirloom quality cradle with a plywood bottom. A much better alternative is to make the bottom using a hardwood, and allow for cross-grain wood movement to avoid damage to the bottom.
For this cradle I milled ¼-inch boards, and used shiplap joints (Note: each division on the gnomon is exactly inch):
The shiplap joint consists of rabbets on adjacent boards, such that each rabbet overlaps the one below. The exposed board edges will be separated by about 3/32-inch when installed; this will allow for seasonal growth and shrinkage, as the wood absorbs and loses moisture during periods of high and low humidity in the region.
The following photo illustrates the appearance of the bottom after creating the joint:
Note: As with drawers, the boards are oriented across the width of the cradle. From the photo above, growth and shrinkage will occur along the length of the cradle, from head board to foot board. Therein lies the importance of the shiplap joint and wood movement.
With the foot board removed, the following perspective shot gives a better idea of the appearance of the bottom in the cradle:
I am looking forward to the glue-up, so I can start work on the rockers. Only then will I begin to consider application of a nice varnish finish on the cradle.
— Al Navas