Wirst wrap the scions in parafilm to keep the moisture. Them paint the graft with chalk, this reflects the sun and keeps it cool. Once the scion starts to grow, the buds will push through the pafarilm and chalk and should handle the sun and heat well.
BTW, when you put a plant in ground (move it from the green house to outdoors) it is a good method to paint the whole plant with chalk in order to protect from hot and dry climate. I have written before about in the forum...