Build the Changing Table Box
The changing table box will have four sides and a plywood and metal bottom. The frame dimensions will match those used for cutouts in the wall. So, if the hole you cut in the wall is 14" tall, cut the frame to 14".
Cut 1"x6" pine to length on the chop saw. For finished corners, cut the ends at 45 degrees. You should have two long and two short pieces.
Using a table saw, set the blade height at a 1/4". Rip a 1/4" dado 1/2" from the bottom, along the length of the box pieces. (The guard on the table saw must be removed to make the cut. Be sure to replace the guard prior to using the saw again.) This dado is for the base of the box to slide into.
Glue and nail three sides of the box together, one long piece and two short. Make sure the dado grooves line up and that the corners join properly.
Cut a piece of sheet metal about 1/4" larger on all sides than the inside dimension of the box. Cut a piece of 1/8" plywood to match. Glue the metal and plywood together and slide the two into the dado with the metal facing up.
Place the last side of the box on and nail. Lay the box flat and use a router with a 1/4" roundover bit, to router the inside and outside edges of the box.