The heater box is covered with a vented cover. The cover is important. It is over lots of hot 220v wire and the heat from the back of the heat tiles must escape. It’s also important that things ( spiders, cats, mice, etc.) and dust don’t accumulate on the back of the heating panels. This could cause a dangerous fire hazard. If I let the machine set for anytime at all I remove this cover and blow all the dust off both sides with the air compressor.
I revised the design a little adding support and for efferent use of materials. My design is made of 4 pieces allowing the top cover to be made from two 2′ x 4′ sheets of ply instead of requiring a full 4′ x 8′ sheet. The end to end size is 52″, just over the 48″ width of a sheet. I couldn’t see buying a whole sheet for just 4″ so I added a 1×6 seam down the middle with from some scrap from one of the earlier steps. This makes the top stronger and saves half a sheet of 1/4 ply. My hope is the added strength will prevent sagging. Since the cover is over lots of hot wire and exposed to heat be sure to seal the wood on both sides and use Hi Temp paint on the vents.