Making the food section smaller using styrofoam will NOT solve your issue.
Fridge is 'chasing ambient air temps'........
Food section cools down late afternoon/evenings/early mornings when ambient air temps are down and then food section temps increase during mid-day when ambient air temps rise.
That is a ventilation issue. Insufficient air flow across cooling unit due to improper installation/baffling/clearances etc.
Also a set of 12V DC thermostatic controlled pan-cake fans installed just below the low temp evaporator (freezer) blowing up with thermostat installed on condenser will help move air across cooling unit.