1. Cut 2 3/4″ plywood @ 30 1/8″ x 50 1/4″.
Make pocket holes on all 4 sides and secure the middle seam together:
3. Build base support frame separate using 1x3s then secure it to the plywood top with pocket holes in the 1x3s:
4. Place the Besta shelf in position to be sure the 1×3 apron piece will fit with the leg in position (but not attached):
In the picture below the legs are NOT attached, just in place: