I designed a simple glass holder for my Anet A8. I am using 4mm glass sheet (should still work with 3mm) and these work great so far. This is my first design project so any feedback is appreciated. I recommend using slightly bed level screws or you'll be compressing your springs a lot.

Your glass must be at-least 7 in by 7 in for this to work 
Use any size M3 screws to adjust tension on glass - you will need 8 screws total

Recommend to be printed in ABS. If you only print PLA and not ABS then PLA might be okay. Printed without any supports.