I was thinking the same on the green lightI cut the plastic sheet down to 40-1/2" X 15-3/4".
The inside edges of the metal overhead shelf are rolled so there is some variability to the measurement. I'm sure my "straight" cut also probably varies a good half-inch as I just winged it with my circular saw and didn't bother with a guide.
Since those posts I have painted my Bambed base pieces with several coats of black paint and Minwax Polycrylic Crystal Clear Topcoat, re-painted and topcoated the edges of the Bambed platform pieces, and topcoated the underside of the platform pieces (didn't bother to paint the undersides black). So now the Bambed wood is more protected than the exterior of my house. I also completely disassembled that neon green magnetic light and painted the green housing black because the green was so out of place.