Soft polenta won't stay soft. The grains seem to keep absorbing the liquid. I found that preparing into a tray, cutting it into a shape, and either frying it, or, putting it into the oven to re-heat and brown was the best practical option. That way, we also could prep that up for 2 days in advance, as it keeps in the fridge. the book "Food Science" by H. Mcgee might be helpful to you.