I am planning to do the same thing. I think the easiest way to do it will be to use thin ABS plastic sheets and screw them to the cross braces normally hidden by the headliner. The edges of the ABS can be shaped by heating with a heat gun and will hol dshape when cool. Then, like Brett said use the Super Trim adhesive to attach the foam to the ceiling, then trim your design in the foam, then coat with adhesive and put your fabric up. You want to make sure and use light fabric so it adheres well and doesn't sag. I should be tackling the same thing in a couple weeks.