Obviously everyone's bath tub may be different sizes so dimensions here won't be too useful.
I cut the pallet wood to the width of the widest point of my tub. I measured the width of the tub at the wall, and the width of the tub at the entrance side. Hence the two different sized gaps on each end of the tray.
I made these gaps so that I could put the support bars inside the tub to dual purpose as a way to keep the tray from sliding in our out of the tub to the left or right.
The diagonal bar is purely for decoration. It definitely looks better with it on there.
Since there would be some water contact I used brass screws to avoid rusting. However, I have not put any polyurethane on it yet. She hasn't used it yet either so I have time :)