mdf radiator shelf bending

Nicola | October 30 2020 | L21 6QB

Hi, I have just purchased a 1200mm long radiator shelf, simple brackets and slots behind the rad. It keeps bending/lifting in the middle when the rad is on. How do i stop this? I dont want to put heavy items on it. Should i put a timber 'splint' underneath so it will resist lifting in the middle?