Probably heavy/double cream is the easiest along with finding ways to add extra food e.g. snacks on breaks, etc.
If eating large portions is a problem, consider a 'little and often' approach.
With double cream I find it easiest to just blend around 200-300ml with some ice cream and a drop of milk using one of those small 'smoothie blenders' (because it's easy to clean). Just make sure to have eaten something 'solid' first to avoid....complications.
1 year