The problem with buckling wax foundation is something I have not had
happen.  Yet 2 of the replies have mentioned it.  Harry mentioned that
it has happened to him.  The temp you mentioned Harry of 25 C is a
normal summer temperature here in South Eastern Ontario Canada.  In fact
alot of North America is experiencing a heat wave right now.  We have
had temps here that ranged from 30C to 35C for weeks on end and some of
the southern United States have been worse.  When I put my super on for
cut comb the average temp was around 25 to 30 C.  None of the foundation
distorted that I could tell.  I wonder if there is something different
about our hives or the foundation?  All of my hives have ventilated
inner boards.  I drill some 2.5 - 5 cm holes and cover them with fine
screen.  I then make a small box (much like a shallow super) and put
some holes in it of the same diameter and cover them with screen.  I
place the box on top of the inner cover and the hive lid on top of the
box.  I've always been a big believer in good hive ventilation.  Maybe
this keeps the hive temperature low enough so as to prevent warping?  I
use 4 pins in every frame to hold the wax.  All my hives get full sun.
Interesting differences!

Kent