Dear scientists

I'm trying to make an experimental study on solar air heaters with a selective coating Aluminum abosrber plate with 0.7 mm thickness but I'm facing a problem with the plate where it make a deformation due to heat and made the contact surface between air and absorber plate unequal (Photo 1).

In addition, I need to isolate the area above the absorber plate (Area 2) and the area below the absorber plate (Area 1) so I need to put a type of adhesives between the absorber plate and sides of solar air heaters body (Photo 2) but the adhesive well prevent the absorber plate to expand in the horizantal direction and produce the unequal areas like photo 1. What type of solution do you advice me to over come this problem.

Thanks a lot