After a bunch of research and reviews, I decided to go with the Ultimate Dreams 11" gel mattress from dreamfoam. We are saving for a wedding and currently have a too-small and aging DIY bed I made 5 years ago, so budget was tight but we needed a new bed badly, and I think this one will be great. I will try to get back and write a review on it here once we have a few nights on it.
My question now is for the platform frame. I am going to build it; I have the tools and knowledge, and I have been very pleased with the frame I built for my full mattress, though I did learn a thing or two since then. Like not to use a solid plywood platform.
Third attempt at the question: My current bed frame is about 3in smaller than the mattress in both dimensions, so the mattress hangs about 1.5" over the edge on all sides. This has two benefits; I have never smacked my shins on the bed frame, and it looks cool to just have the bed sort of floating on five legs, as that is the only part of the frame you can see when the bed is made. Is there any reason I should not just scale this up for a king? I am going to replace the plywood with slats, and beef up the design a little to handle the extra span and weight of a king bed, but I cannot come up with any reason why this would damage the mattress. For some reason, though, I cannot find any examples of it anywhere else, so it makes me nervous.
And while I’m asking questions; what would be the difference between standard slats and plywood with a pattern of 2" holes drilled for ventilation? I am leaning toward the slats, but a friend is going to build a bed with me and is thinking of going the hole-ey plywood route.