Apologies as I haven't read any replies yet... I love the idea... but I think the length of your plate is a little long and will not be flat. For that size you need better support reaching nearer to the ends in my opinion.
But I did just what you are proposing... I salvaged an old Craftsman roll away tool chest out of a recycle dumpster, fixed it, and tacked a 4ft long 1/2in plate on top of it. I have limited space and until I can afford the space and the cost, it does fine for small work. I use an auxiliary roller support table for bigger jobs and the real big ones still on the floor unfortunately. Make a custom stand with heavy duty casters and you can maybe build in a tool chest or shelfs or...
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