Another thing I couldn't live without is a 36" metal straight edge; and an 18" one is handy, too. Either a metal yardstick or a length of aluminum "T" or "L" from a hardware store. Preferably both as it is sometimes handy to use the yardstick as a batten in slicing sheeting with a curve (like on a fuselage side).

Making butt-joints for sheeting works better if you trim the edges first with a razor-knife using the straightedge as a guide.