Make each level 1" bigger than the tubs and thats it. If not mistaken i made mine 20.75 wide which is 1" wider than the mortar tubs.

And depth can be 1" longer than tub to clearance. And to make cutting the screen easier, use a circular saw and turn the blade so it spins backwards and cut the screen when stapled to the pieces. I was able to do this all in under 15 mins of stapling and cutting