Depends on the temperature…if it is a warm day or kept in a warm place it can be quite horrible (in my opinion) after a few hours…although I use 50/50 water and plant based milk…may be different with water alone. If a coolish place I reckon it will be OK…altough my personal preference is drinking Huel chilled.
Id do this quite regularly. If I’m going to be out for an activity, I’ll make up my huel in the morning and keep it in the fridge. the I’ll keep it in my rucksack for 4+ hours before consuming it on the tube on the way home. Tastes quite fine to me.
For you, making it the night before would work
If it’s going to get warm in that time you could either start with chilled huel - water in fridge, ice cubes, make up huel night before and leave it in fridge etc
Or you could get a vacuum bottle and keep in at the same Temp that way.