One thing i would suggest is to add some flat white to those walls. This will reflect the light the best, better than mylar even. You can either paint it or get some white cardboard and line it, its needs to be FLAT white not glossy, as it absorbs light.
I figured you might mention that as I haven't finished preparing the space. I'm thinking about some other reflective material like AL foil or one of those Space/Rescue Blankets) or do you consider that overkill?