HarborFreight has aluminum carpenters rafter tri-angles on sale for 1$ with there latest ad coupon /flyer. These are great for squaring up fuselages on the board, making sure stab is level, etc.
Can't go wrong for a buck, the plastic ones at Home Depot are a couple bucks more and they crack, get glue on that won't scrape off, and warp if you leave then in the hot sun.