Some projects just don't lend themselves to conventional clamping techniques. The flag case shown in these photos is a great example. With angled cuts on all three corners, the best way to get clamp pressure directly on the joint is to add clamp blocks.
Using hot glue, fasten a clamp block on the outside face at each joint (Photo 1). The blocks should be cut at an angle that will put clamp pressure perpendicular to the joint. Make sure the blocks are evenly spaced from the end of the joint.
Spread glue on the joint (Photo 2).
Close the joint by clamping across the blocks (Photo 3). If the blocks were evenly spaced when you glued them on, you'll have uniform pressure on the joint (Photo 4).
After the glue in the joint is dry, snap the glue blocks free of the project. Remove hot glue residue using a chisel (Photo 5).