3018 CNC Vise

Not quite designed to be used the way the OP designed his however it is derived from his design. I made this vise to be used with my 3018 wasteboard with the holes being a very specific spacing.

Link to my etsy if youre interested - https://www.etsy.com/your/shops/me/listing-editor/edit/1556130408

I use all M5 hardware (no nuts) and designed the holes to be small enough for the screws to grip the plastic. In my pictures you'll notice the middle bar has holes however I have since corrected that and uploaded a newer version without the holes as they are unnecessary.

I've tested this on my machine and with the proper tension on the screws, the clamping pressure is enough to hold the material safely in place while the machine is active. You could also use sandpaper as an extra method of gripping power if your material is smooth against the smooth plastic and sliding, I've used this on a different print on a similar design and works great.