Ceiling tiles on top of a flat surface(glass, spare formica counter top, new door from local Lowes/HOme Depot) will work. JUs take a good three or four foot level and check your flat surface from many angles to insure it really is flat. Pins work fine. Go to Walmart, JoAnn's, etc. for good deals on boxes of different size pins. Put wax paper between your plans and your balsa. The wax paper makes removing the built part easier to remove. Your built part doesn't stick to your plans.