Sounds like a pretty good idea, especially in terms of pure mass and solidity.
This might provide one with a very stable work surface, if properly supported and reinforced. The problem with concrete, is weight.
Your formwork would have to be quite accurate, and stable to allow the concrete to cure for at least 21-28 days. I would use a 5000 PSI mix design, and an intermediate size aggregate, 1" and smaller. Perhaps 1 or 2 layers of (4x4 to 6x6) x .25 in. WWF could be emdedded therein, depending upon total slab thickness.
1- Don't forget to vibrate as you pour, to remove voids. ( Caveat: too much vibration leads to segregation, and loss of ultimate design strength.)
2- Do not make your mix too wet. ( Caveat: too much water in the mix increases the water to cement ratio, thereby reducing ultimate design strength.)
3- Once your concrete has set, ( usually 24 hrs.) cover it with plastic, and mist with water once a day.
4- Protect the young concrete from freezing for at least 7 days after initial set.
You could probably make a template to hold a series of threaded rods/studs, and mount them in the freshly poured concrete. Aluminum, is a bad choice for this because it reacts with cement paste. Concrete and aluminum, just don't get along. Steel would be a better choice, stainless even more preferrable.
All of this information can be found in your CRSI ( Concrete Reinforcing Steel Institute) manual. Probably available at the library, or at the home of an engineering friend.
As for table size, 4'x8' would lend itself to utility of materials. One might also consider putting a very small arch in the formwork to allow for deflection when placed into operating position.
I would probably try something like this, but I have lathes, drills, mills, welders, plasma torches, grinders, and a plethora of other tools with which to work. As a bridge engineer, I've seen concrete, both good and bad.
Good luck !!