I found that I like flat white paint as well as anything. I started out many many moons ago with mylar, which I came to hate. It is simply so hard to work with, does not clean well, and only has those great reflective numbers if it is hung straight with no wrinkles (almost an impossibilitgy). From there, I went on to panda film. I liked it, but you still had to attach it t the walls somehow, leaving nail/staple holes and/or tape residue. Now, I just keep flat white paint around. It is incredibly cheap, washes well, is available everywhere, and is so easy to deal with when you need to take a grow room down . I am still cussing some self stick mylar that was in a grow closet I recently had to turn back into a real closet.
I am not sure what you are planning on using, but keep in mind that just bcause something looks reflective does not mean it is. For instance, flat white paint is far superior at reflecting light back than glossy paint. Foil is also a very poor reflector of light. So before you buy something that. might look reflective (but might now be), make sure that it is going to do what you want it to. I would suggest passing your ideas by the people here to be sure you do not buy something that looks reflective but is not.