Once you have the materials it's probably about two hours to cut and assemble, then another hour to get it mounted on the wall.
Get the lumber yard to rough cut pieces close to the dimensions you want, this will reduce the amount of time you have to horse the plywood around on your table saw at home.
The tedious work is getting it mounted on the wall while keeping it square. Mine were made out of 3/4 inch plywood throughout. Applying the silicone and keeping it off the exposed wood that you intend to apply stain/varnish to is also troublesome. Wood that has been exposed to silicone will never take stain well, it shows up as a smear, much like a glue squirt that didn't get cleaned up.