Make a cheese sauce that doesn't go solid when cooled The way I normally go about making a cheese sauce for something like macaroni cheese, I start with a roux, slowly add milk, heat to thicken then add the cheese. This works just fine and is great if the food is being eaten immediately.

Recently I've taken to batch cooking my meals for the week, so I'll cook up 7 portions of a meal and have them over the course of the week.

When I've tried to do things like macaroni cheese or a carbonara, once the meal has cooled in the fridge it becomes a solid block that's hard to serve and never really goes back to what it was when it was fresh if it's reheated.

Is there a method of creating a cheese sauce which, either doesn't solidify when it's cooled (more viscous is okay, i just don't want it to be a solid block) or at least will go back to being closer to what it was fresh, once it's been reheated?
Your sauce is thickening up too much because it's losing moisture. After you cook your cheese sauce to perfection and mix it in with your pasta 2 things are happening:


The sauce loses moisture due to evaporation. It's hot, and even with the lid on you will still lose moisture
The pasta will soak up water from the sauce. It's just starch, and starch sponges water up. After being mixed together the pasta will continue to soften by pulling water from the sauce


The solution is easy - add more water to replace the lost moisture while it is still warm. When I make this sort of thing I let it cool somewhat and then add pasta water in small amounts until I get a slacker sauce, then I refrigerate or freeze it. Once you get the idea of how much extra moisture you need you can add some of it to the sauce to begin with, but reserve a bit for after cooking as changes in pasta types and how done it is makes a difference in how much the pasta will soak up. 

I have also had some success in rehydrating too thick mac and cheese by reheating it partially mixing some water or milk in, letting it sit a couple of minutes, then reheating the rest of the way slowly.