Hey Dan... welcome to the forums.
I'm going to make a few suggestions, take them as you will.
I'm going to use 3 sections of 6ft long and 12inches wide and 2ft high, just for explainations sake. If you want them to be longer or wider or whatever, that's fine, just substitute numbers! Oh, you'll notice I like things to be sturdy... very sturdy. I've had boxes fail under me and that's not fun.
Start by builing six 6'x1' rectangles (measure from the OUTSIDE of the 2x4's. Screwing them together is better than nails. After those are built, cut eightteen 2' long peices of 2x4. Using six pieces of the 2' sections, connect two of those rectangles together, 1 piece in each corner, 2 pieces in the center. Extra support is good. Okay, so the structure of your box should be built. Now cut some plywood to fit on the top of each box. If ya didn't mess up it should be 6'x1',
. Now here comes the expensive part. Lexan will be your topsheet... the stuff that'll let you slide. It's really expensive... $200+ for the amount you'll need. There are other cheaper options... let me know if you want those. They work just as well, just may not last as long.
Since you're thinking of flat downs, etc (battleships are fun), all you have to do is angle the corners in on one or two of the boxes, not hard.
I'm sick, so this probably looks like mumbo jumbo, but just let me know if you've got questions.