Mine is a torsion box. I worked on it being dead flat and it won’t warp AND is very solid. Grid is 3" with 2 - 3/4" skins. I made the surface so it was the same height as my table saw and work bench so that I can use them as a support table when I need to index for larger signs. And on four casters that double lock.
Here it is without the top skin:
Nice setup! Very jealous. If only it was black and yellow then it would be perfect. 
Ah! Makes perfect sense now! Awesome 
Finally got the Table done and Shapeoko wired up. The main hardness that came with it has minor alligator clip missing/damage and Carbide 3D overnight me a new harness!! What a customer service.
Hopefully getting coolant soon and few mod on my 220V circuit and we’ll be ready for to fire it up.
I used a solid core door for my 2x4 machine.
Awesome!
Question. I see it’s on casters. Do you pull it out into the room at all? Also, if not, do you find that you ever have to get to the other end of the 4" table? I’m trying to decide where my final set up is going to be and feel like I will need full access all around the table. But if you haven’t found the need, I will feel more comfortable NOT.
I definitely think the base should be mobile. I have a set of retractable casters on mine and its a love/hate relationship. I love being able to move it and I hate disengaging the casters when I need to. I have pondered better approaches for a while and never came up with a really great idea.
Today I ran across this video. The guy talks about the ones I have and why the suck and then goes through his process for a better plan. I am not sure his approach is scalable for a table large enough for a 4x4 machine, but its worth considering.
I watched another of his videos on a “better” tablesaw sled and like it as well.
I used these style on my build above. Love it! These are little bit overkilled for my setup but the price different is negligible. Once you level the pads and all 4 are down, Table is solid.
I have those on my lathe but I have to lay on the floor to adjust them.
I used these double locking casters. I’ve been pretty happy with them so far.
