That's a cool idea for a homebrew cover. I'd use at least 1/2" over the 1/4" suggested above but it could work. It could be quite secure if done properly. Only downside is you would have to drill holes to screw the channel into place and it would reduce the bed width from above.I have seen similar things on youtube. I saw one guy who got some aluminum U chanel he attached to the sides and he just slides the plywood into it.
https://shedheads.net/wp-content/uploads/2018/11/DIY-640x480.jpg
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