We took the top and bottom shelves out of the changing table, including the side supports.
I took the centre shelf out, cut the magnetic chalkboard roll to size and stuck it to the centre shelf, then put the centre shelf (now chalkboardy!) back in place.
We reassembled the back supports on both top and bottom shelves. For the bottom shelf, the front support was turned at a 90 degree angle and screwed in using its original bolts into a pair of holes that existed to hold the side pieces on about 2/3rds of the way to the back. This provided structural stability.
My husband drilled a hole through each side about 2/3rds of the way to the back on the top. With the front support for the top shelf flipped upside down, the groove to hold the shelf was perfectly level with the groove on the back support.
My husband cut one of the shelves to fit the new 1/3rd depth shelf on top and reassembled the desk for the final time.
1. You could do this without the drilling or cutting if you forgo the top shelf — just screw the front support into the side holes left by the side supports. It won’t be level to insert the shelf, but it will enhance structural stability with less effort.