use dzus buttons, nutcerts can spin/come loose and then you have problems. In reality, bending the bottom up on the sides (u-shape) and laying the side panels over, dropping it 1/2" is not a big deal. and will not affect the looks. This is a complete new build, you can do it many different ways and will be period correct. The trick is not to over-think it and do it as simply as you can---that's the way it was done back in the day