Keeping a mitred joint fixed in position when a bench surface is not available is a real challenge. The tried and true solution has always been to use pointed spring clamps, which are, unfortunately, mighty scarce these days.
We've found a manufacturer right here in Ohio, along with a special spreading pliers that makes applying these spring-steel clamps very easy. The tiny "dimple" left by the points is easily filled. The Clamps themselves come in sets of four. No matter how many sets you get, you only need one pair of Positioning Pliers.