That's really cool. I have a couple of questions because I'm about to embark on two similar projects.
1. What kind of foam did you use? That looks like the thickness I'd like to use for my part time camper conversion I'm making for my minivan.
HD36 firm, high density upholstery foam, 6" thick. it was very very expensive, even on sale, but has a life expectancy of 15 years as opposed to 5ish for the cheaper stuff. Not pictured, but before I cover them they'll be wrapped in polyester fiberfill for better looks & softness. TBH the thickness is probably overkill for this firmness; I only sink down about halfway & can't feel the hard bottom at all. If you went with anything softer and didn't have a webbed/spring bottom (as furniture usually does, but van conversions usually don't), 6" is probably necessary to not feel like you're bottoming out.
2. What kind of support does a concrete table need underneath? I remember from my construction days that when we would lay slabs of marble in bathrooms (very high end houses) we basically needed continuous support as the marble was prone to breaking like glass.
I'm going to make an island on castors for my patio that I'd like to put a concrete top on. I figure that fact that it will move will mean that it needs to be extra thick and well supported. Do you think I can get away with 1x every 12" or so underneath or should I got with continuous support? I'm also assuming that you just attached the top with construction adhesive, or did you use tapcons?
The only supports are the two 2x2's running down the length, so it has a 16" unsupported gap all the way along. no internal support (rebar/wire mesh) either. I jumped around on it to make sure it was solid and nothing broke, so I guess it's good. Based on this, the 1x every 12" would work (assuming thickness >1", which is where I'm at). Probably the thinner you go the more support is needed?? I'd love to be able to give you advice on how much support is actually right, but I'd really just be guessing.
The top is just sitting on the base right now, no fasteners/adhesives, and I think I'm going to leave it that way. It's heavy enough that it's definitely not going anywhere, and being able to move the top separately makes the weight a lot less awkward to carry by myself.