Here are assemble aids that hold those right angles together until the glue dries or you can secure corners with screws or nails. We recommend the models below.
Wise Buys: Right-angle Assembly Squares
POWERTEC 71124 90 Degree Positioning Squares, Woodworking Tools for Right Angle Corner Clamps, Carpenter Tools for Cabinets, Picture Frames and Boxes, 4 set of 4
Wise Buys: Mini-Lathes
Clamping Squares - Lee Valley Tools
Clamps and Clamping
Wise Buys: Mini-Lathes
miter square Draper 18952 Try and Mitre Square, 150mm , Blue
Buy VEVOR 90 Degree Positioning Squares, 5.5 x 5.5 Right Angle Clamps for Woodworking, Aluminum Alloy Corner Clamping Squares for Boxes Cabinets
VEVOR 90 Degree Positioning Squares, 5.5 x 5.5 Right Angle Clamps for Woodworking, Aluminum Alloy Corner Clamping Squares for Boxes Cabinets
Wise Buys: Right-angle Assembly Squares
Milescraft 8" ClampSquares - 90 Degree Corner Clamp, Positioning/Assembly Squares (Set of 2) 4011 - The Home Depot