Rip pieces of 3/8" or 1/2" plywood 1 1/4" and 2 1/4" wide. You'll need 6 pieces of each width roughly 20" long. Later we'll cut them down to 19 1/2" after they are fastened together. Take a 2 1/4" piece, apply wood glue to one edge, and nail an 1 1/4" wide piece to it (pic 3). This creates the six 20" angled side pieces which are 1 1/4" x 2 5/8" (pic 4). Trim the ends even and to a length or 19 1/2". To make an isosceles triangle we need to trim 15° off the ends to create 30° angles. On my miter saw, I use an auxiliary fence which is basically a large 90° angle (pic 5). This fence allows me to accurately make this cut with my miter saw set at 30°. To cut the other end of the side piece, use a 2x4 under it for support. You could also make this cut on a table saw, hand saw, etc. Don't forget the ends are cut in opposite directions.
Next you'll need band clamps to hold the triangles together after gluing the edges (pic 6). To support the corners, glue and nail a small triangle across the bottom of the corners. Picture 7 shows the finished triangles ready for use. You can imagine how the edges of the frames hold the table legs in the correct position.