You can get Masonite in 1/8" and 1/16" thickness. The thinner stuff would be fine for a back. The trouble is getting a pattern of slots and holes to look good and even. Takes some careful layout and cutting. The easiest for tube cooling is a slot across the top and one across the bottom of the chassis area, allowing for air in, air out. Here's an example of one I did in the thin Masonite for a table set.
