Jtech sells OD3 acrylic that’s not terrible priced. It’s not exactly the highest protection rating but is OK for stray light, and certainly for observation with supplemental glasses.
First, get to know your laser wavelength. Then, ask the manufacturers for data about absorption values on that specific wavelength.
For example, for a 450nm diode laser, orange or red acrylic has good absorption values. This is one document I found about the acrylics I checked for my enclosures. It’s in German, but I think you get the message