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.
WETOLS 12PCS Positioning Squares, 90 Degree Right Angle Clamp Woodworking Tools for Handyman, Wood Clamp Square Clamp for Carpenter DIY Pictures Frames, Cabinets, Boxes, etc - WE867
WETOLS 12PCS Positioning Squares, 90 Degree Right Angle Clamp Woodworking Tools for Handyman, Wood Clamp Square Clamp for Carpenter DIY Pictures Frames, Cabinets, Boxes, etc - WE867
Here's a cute trick for clamping acute angles
Clamping Squares - Lee Valley Tools
Homemade Hold-downs
Clamps and Clamping
LEGO Creator Expert Assembly Square (10255)
Silginnes 90 Degree Positioning Squares Right Angle Clamps for Woodworking, 4 Pack(5.5''*5.5'')Aluminium Alloy Corner Clamp Woodworking Carpenter Clamping Tool Picture Frame Box Cabinets Drawers Red, Angle Clamps - Canada
in Angle Clamps.
Aluminum Right Angle Clamps for Woodworking, 90 Degree Positioning Squares Carpenter Corner Clamping Square Tool for Picture Frame Box Cabinets
WEN 12 in. 300 mm Right Angle Heavy-Duty Carpenters Square
WETOLS 12PCS Positioning Squares, 90 Degree Right Angle Clamp Woodworking Tools for Handyman, Wood Clamp Square Clamp for Carpenter DIY Pictures Frames, Cabinets, Boxes, etc - WE867
Wise Buys: Mini-Lathes
HARDELL 90 Degree Positioning Squares, Right Angle Clamps 5.5 x 5.5(14 x 14cm) Aluminum Alloy Woodworking Carpenter (4 Pack), Carpentry Squares - Canada
Here's a cute trick for clamping acute angles
Buying good tools cheap #2 - The combination square - Paul Sellers' Blog