Cutting 3.5mm wide grooves into wood strips that can hold 3.2mm hardboard sheets
Making ends 45 degree angled for top and bottom strips
Gluing strips and 3.2mm hardboard together
Making sure corners are snug and in right angles
Trying not to use too much glue that will overshoot
Adding a transparent window
Ready for use
Disclaimer: This is actually a cover for my CNC machine. Reducing noise and dust.