Heat reflectors for radiators, a cheap 'add to' for the home energy DIY list, as often or not the combined use of energy / heat saving ideas will impact more than a single addition, this i do consider a worthwhile idea whether you make it yourself of use a commercial ready made product.
To make simple reflectors for your radiators you can use some free / reclaimed materials.
1.Used packing material (polystyrene)
2.Cooking foil
3.Double sided tape or glue
4.Discarded vape liquid caps
Be aware some glues are not suitable to use with polystyrene.
The idea of these things is to reflect heat away from the wall as opposed for it being to being absorbed into it. Below shows thermal imaging of an example.
Commercial available reflector results
radflek
Here is our DIY method. Cut the sheet to fit behind your radiator, you may need to size more than a single piece due to the brackets holding the radiator to the wall, just ensure they butt up close upon fitting. Wrap the sheet in cooking foil, use either double sided tape or glue although be aware not all glues work with polystyrene. Fit behind radiator, i spaced with used vape liquid tops which fitted just right but again you could use glue or tape to stick to the wall. 
LINKS
Any heat reflective material could be utilised such as a car windshield sun reflector.
