I've built a couple tables like this and they work great. Mine sit on sawhorses so they can be taken down for storage off-season. I put the styrofoam on top and pinned the heating cable to it with little staples, and used a couple buckets of pea gravel instead of sand. And I guess I'm old-school but I used a thermostatic controller from Grainger to control the temp. Does it really take a microprocessor?
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A little simpler