fence expansion causiing trim to pull off

Put up panel gate (japanese gate) made from cedar, stained with water proof sealer. Looks great.
Problem is that the panel expanded (duh) and pulled away from the gate fram (iron pieces with 2x4 cedar). I can fix the expansion problem by sawing the panel into strips and attaching then individually, but close together so effect is very close to panel look.
BUT I also have some cross pieces of cedar that are meant to go at the top and botton of the fence as trim, stained to match the posts. How do I attach these to the slats without having the same problem - either the slats expanding or the cross pieces expanding?
I know the typical fixes for furnture, but this is a gate and typical does not work..
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