Home depot and Lowes sells Laminated Melamine Shelves that are 16" deep and can be had in 4', 6', and 8' lengths for less than an Andrew Jackson. Cut it to the length you need and seal the edges. Cut holes in the top of the proper size for the light, suround the opening in Aluminum HVAC tape and then staple a screen down that is the right size and cover the staples with more of the above tape.