All you do is score the plastic a few times with this special knife and then break it on the score line. Here I'm using a straight edge to help guide my knife. Then I clamped the plastic between two pieces of wood and broke it. This method works good.
 

Here I'm gluing the frame together using my new strap clamp made be Bessey. This type of clamp comes in very handy for gluing four sided boxes or just about anything that has sides. And even round stuff. The strap is 23 feet long and comes with four 'clamp-corners' that help hold each corner of your project. And Bessey claims this clamp will exert  2300 pounds of force.
 

The clip corners (right) with twin-tilting pressure pads auto-adjust from 60° to 180° for easy assembly of most multi-angle projects, and came with the strap clamp. However, I also bought a different style that works even better, which are made by Lee Valley (left).
 

These Lee Valley variable-angle corners will clamp joints ranging from 30° to almost 180°, such as polygon frames or triangular bases, like I'm working with. And with the 45 degree angle that my flag display case has, the smaller Bessey pieces won't conform to the angle I needed. The backs of both clamps have notchs that holds the band strap in place while clamping. Or you can just use the band only if your project doesn't need the corners.
 

I'm sanding everything with 150 grit paper, including the three angles to soften the edges. This process goes fast and now it's time to drill some holes.
 

I used some dark stain on the pine case but not the black walnut.