I would suggest that you re-think the frame. I replaced the A/C on my 75 TW and boxed the hole with some wood pieces that another member fabricated for me.
The new A/C is secured by clamping the two halves of the unit to the outer and inner skins of the trailer. Without putting something solid between the halves the skins are going to buckle and you will not get a good seal to the roof.
Adding an extra layer of aluminum will not help when you tighten the bolts that clamp the A/C top and bottom pieces together, IMO.
WBCCI President 2016-2017